<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2024 Workiva-->
<!--r:31b428d4-e9c1-4ab3-a548-aa5f0ca880f2,g:59bdcab4-9162-4407-b5cf-b8585933ca5f-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://quantum-si.com/role/Cover" xlink:type="simple" xlink:href="qsi-20240930.xsd#Cover"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityListingsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsTable_1ad28626-461f-42ec-af86-1cf579428109" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityListingsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityListingsTable_1ad28626-461f-42ec-af86-1cf579428109" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_ce341e5a-f0cb-4f03-838e-563681f9d089" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntityListingsTable_1ad28626-461f-42ec-af86-1cf579428109" xlink:to="loc_us-gaap_StatementClassOfStockAxis_ce341e5a-f0cb-4f03-838e-563681f9d089" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ce341e5a-f0cb-4f03-838e-563681f9d089_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_ce341e5a-f0cb-4f03-838e-563681f9d089" xlink:to="loc_us-gaap_ClassOfStockDomain_ce341e5a-f0cb-4f03-838e-563681f9d089_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c1318f61-3d8a-4239-bf27-e12760c39ee9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_ce341e5a-f0cb-4f03-838e-563681f9d089" xlink:to="loc_us-gaap_ClassOfStockDomain_c1318f61-3d8a-4239-bf27-e12760c39ee9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_39fbe273-9379-4840-969f-8e0588c4f191" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c1318f61-3d8a-4239-bf27-e12760c39ee9" xlink:to="loc_us-gaap_CommonClassAMember_39fbe273-9379-4840-969f-8e0588c4f191" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_RedeemableWarrantsEachWholeWarrantExercisableForOneShareOfClassACommonStockEachAtAnExercisePriceOf1150PerShareMember_7ea0c16e-4efb-432c-b4a7-e320d47c1c95" xlink:href="qsi-20240930.xsd#qsi_RedeemableWarrantsEachWholeWarrantExercisableForOneShareOfClassACommonStockEachAtAnExercisePriceOf1150PerShareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c1318f61-3d8a-4239-bf27-e12760c39ee9" xlink:to="loc_qsi_RedeemableWarrantsEachWholeWarrantExercisableForOneShareOfClassACommonStockEachAtAnExercisePriceOf1150PerShareMember_7ea0c16e-4efb-432c-b4a7-e320d47c1c95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_626c1811-8e5c-48ef-b83a-cd039b728968" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c1318f61-3d8a-4239-bf27-e12760c39ee9" xlink:to="loc_us-gaap_CommonClassBMember_626c1811-8e5c-48ef-b83a-cd039b728968" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_87ed536d-ac96-44e1-a55c-8f3f4914da89" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_DocumentType_87ed536d-ac96-44e1-a55c-8f3f4914da89" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_eaccf26b-41cc-40d6-96aa-a33b866aca6b" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_DocumentQuarterlyReport_eaccf26b-41cc-40d6-96aa-a33b866aca6b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_c30c9ace-84a3-4720-bd2f-9a34ab5ce3d4" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_DocumentPeriodEndDate_c30c9ace-84a3-4720-bd2f-9a34ab5ce3d4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_2dadc02e-6c26-4722-ba29-ce9949b5b172" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_DocumentTransitionReport_2dadc02e-6c26-4722-ba29-ce9949b5b172" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_ea7c13d9-2eb6-4fe3-9671-dbd7df78c988" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityFileNumber_ea7c13d9-2eb6-4fe3-9671-dbd7df78c988" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_d568aa5e-8f10-40cf-b9bb-9aa19c9d499b" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityRegistrantName_d568aa5e-8f10-40cf-b9bb-9aa19c9d499b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_2506331d-2dad-41d0-83b5-157bb74b7801" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityIncorporationStateCountryCode_2506331d-2dad-41d0-83b5-157bb74b7801" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_ca7b0789-dbee-4e92-9a8d-e25920793832" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityTaxIdentificationNumber_ca7b0789-dbee-4e92-9a8d-e25920793832" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_f0d6a7a4-2006-46ab-87c1-4f5a80e600ff" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityAddressAddressLine1_f0d6a7a4-2006-46ab-87c1-4f5a80e600ff" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_da6b7814-4872-4e18-a612-7ffe1393cb0b" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityAddressCityOrTown_da6b7814-4872-4e18-a612-7ffe1393cb0b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_87b04213-e0f9-4db3-a51d-9110878bae78" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityAddressStateOrProvince_87b04213-e0f9-4db3-a51d-9110878bae78" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_e8b1ac89-a7ea-4c1f-9161-3626d533dbcb" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityAddressPostalZipCode_e8b1ac89-a7ea-4c1f-9161-3626d533dbcb" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_e9a1c9d7-ba07-4e40-bc2e-cca7262d0d63" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_CityAreaCode_e9a1c9d7-ba07-4e40-bc2e-cca7262d0d63" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_82476adb-8c86-4c51-ad10-af5c62972af6" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_LocalPhoneNumber_82476adb-8c86-4c51-ad10-af5c62972af6" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_fca7f283-9617-4056-b343-0725d7ed371f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_Security12bTitle_fca7f283-9617-4056-b343-0725d7ed371f" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_afab9218-e844-4013-b75d-b28adbb41ec7" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_TradingSymbol_afab9218-e844-4013-b75d-b28adbb41ec7" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_da0b0b9b-b49e-406a-b5ba-93589dd0faf4" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_SecurityExchangeName_da0b0b9b-b49e-406a-b5ba-93589dd0faf4" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_b07d796c-0023-4026-aa22-e30709b27a1f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityCurrentReportingStatus_b07d796c-0023-4026-aa22-e30709b27a1f" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_20bf9f95-79b3-4bb1-91da-b83f3be69194" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityInteractiveDataCurrent_20bf9f95-79b3-4bb1-91da-b83f3be69194" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_df500315-f4f0-419a-890f-6edab58805c1" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityFilerCategory_df500315-f4f0-419a-890f-6edab58805c1" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_0ec187c7-3e9a-4b86-b10c-106058c7cb9c" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntitySmallBusiness_0ec187c7-3e9a-4b86-b10c-106058c7cb9c" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_197c772e-8ea5-4fa1-8569-40ea606b31aa" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityEmergingGrowthCompany_197c772e-8ea5-4fa1-8569-40ea606b31aa" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_4887c242-e1cf-4a41-b11b-bbd412ffe100" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityShellCompany_4887c242-e1cf-4a41-b11b-bbd412ffe100" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_8e89b048-161c-4f89-9e3f-f64e589f06c0" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_8e89b048-161c-4f89-9e3f-f64e589f06c0" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_e10f0378-6fe1-420e-9cdb-0360af5729fc" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_EntityCentralIndexKey_e10f0378-6fe1-420e-9cdb-0360af5729fc" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_7495ac61-c791-4736-a526-3fefca3b926f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_CurrentFiscalYearEndDate_7495ac61-c791-4736-a526-3fefca3b926f" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_4de3af85-e6d6-4f3b-9480-56799c31d127" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_DocumentFiscalYearFocus_4de3af85-e6d6-4f3b-9480-56799c31d127" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_2840fa34-851b-4e32-aad4-f4bad1f24663" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_DocumentFiscalPeriodFocus_2840fa34-851b-4e32-aad4-f4bad1f24663" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_d006bcb2-bc87-4ba3-8738-4ddac3e746b3" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_85555aad-2afe-4983-888d-28ec587ff6c7" xlink:to="loc_dei_AmendmentFlag_d006bcb2-bc87-4ba3-8738-4ddac3e746b3" xlink:type="arc" order="28"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="qsi-20240930.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETS"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_a28096ba-b3de-4b7e-b10a-c854e7067d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_aa24c493-b34c-48dd-8440-1683019e46b5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_a28096ba-b3de-4b7e-b10a-c854e7067d4e" xlink:to="loc_us-gaap_StatementTable_aa24c493-b34c-48dd-8440-1683019e46b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_34cc43d9-ba75-43ff-b52e-90e2208cde5b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_aa24c493-b34c-48dd-8440-1683019e46b5" xlink:to="loc_us-gaap_StatementClassOfStockAxis_34cc43d9-ba75-43ff-b52e-90e2208cde5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_34cc43d9-ba75-43ff-b52e-90e2208cde5b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_34cc43d9-ba75-43ff-b52e-90e2208cde5b" xlink:to="loc_us-gaap_ClassOfStockDomain_34cc43d9-ba75-43ff-b52e-90e2208cde5b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_19c8d1c2-c794-472b-ab48-69b61f6888fe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_34cc43d9-ba75-43ff-b52e-90e2208cde5b" xlink:to="loc_us-gaap_ClassOfStockDomain_19c8d1c2-c794-472b-ab48-69b61f6888fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_64cdcccd-b09c-4030-bde3-06ba95f32918" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_19c8d1c2-c794-472b-ab48-69b61f6888fe" xlink:to="loc_us-gaap_CommonClassAMember_64cdcccd-b09c-4030-bde3-06ba95f32918" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_a1e9bddc-4911-4fba-a5a5-3a6ba21ba4bc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_19c8d1c2-c794-472b-ab48-69b61f6888fe" xlink:to="loc_us-gaap_CommonClassBMember_a1e9bddc-4911-4fba-a5a5-3a6ba21ba4bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_991cc593-1b90-4dc9-aee2-ef6de45f2a5c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a28096ba-b3de-4b7e-b10a-c854e7067d4e" xlink:to="loc_us-gaap_AssetsAbstract_991cc593-1b90-4dc9-aee2-ef6de45f2a5c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_d7e44e62-e17c-42af-b010-e80e03fabcfe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_991cc593-1b90-4dc9-aee2-ef6de45f2a5c" xlink:to="loc_us-gaap_AssetsCurrentAbstract_d7e44e62-e17c-42af-b010-e80e03fabcfe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_a51fa2fc-56ef-4699-b254-2bc8f25359da" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_d7e44e62-e17c-42af-b010-e80e03fabcfe" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_a51fa2fc-56ef-4699-b254-2bc8f25359da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradingSecuritiesDebt_59a1aaec-acf4-4b17-b1f7-2d3942066b94" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TradingSecuritiesDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_d7e44e62-e17c-42af-b010-e80e03fabcfe" xlink:to="loc_us-gaap_TradingSecuritiesDebt_59a1aaec-acf4-4b17-b1f7-2d3942066b94" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_409d3f25-f344-4d2e-981d-89b409085b81" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_d7e44e62-e17c-42af-b010-e80e03fabcfe" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_409d3f25-f344-4d2e-981d-89b409085b81" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_d78642d2-7120-4b5f-866b-ee3787ae646f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_d7e44e62-e17c-42af-b010-e80e03fabcfe" xlink:to="loc_us-gaap_InventoryNet_d78642d2-7120-4b5f-866b-ee3787ae646f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_646a144d-17ab-43a5-a19a-ab9523e6b8f0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_d7e44e62-e17c-42af-b010-e80e03fabcfe" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_646a144d-17ab-43a5-a19a-ab9523e6b8f0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_98d324f2-cdc8-4bc8-abe8-6d3c5422a351" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_d7e44e62-e17c-42af-b010-e80e03fabcfe" xlink:to="loc_us-gaap_AssetsCurrent_98d324f2-cdc8-4bc8-abe8-6d3c5422a351" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_cc5905b5-d737-4f43-b979-3ff6238f77c1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_991cc593-1b90-4dc9-aee2-ef6de45f2a5c" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_cc5905b5-d737-4f43-b979-3ff6238f77c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_6d018d86-312a-410a-be45-691c29d73a45" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_991cc593-1b90-4dc9-aee2-ef6de45f2a5c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_6d018d86-312a-410a-be45-691c29d73a45" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_fd816683-5a7c-44d4-b341-a06cd5378995" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_991cc593-1b90-4dc9-aee2-ef6de45f2a5c" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_fd816683-5a7c-44d4-b341-a06cd5378995" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_37121eb0-6eb2-42e8-98e3-38d837cf9fce" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_991cc593-1b90-4dc9-aee2-ef6de45f2a5c" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_37121eb0-6eb2-42e8-98e3-38d837cf9fce" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_0caa6ca3-32e3-43d2-acad-209f8c82101c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_991cc593-1b90-4dc9-aee2-ef6de45f2a5c" xlink:to="loc_us-gaap_Assets_0caa6ca3-32e3-43d2-acad-209f8c82101c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a28096ba-b3de-4b7e-b10a-c854e7067d4e" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_8d735daf-da23-4dad-adf8-5fb199611126" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_8d735daf-da23-4dad-adf8-5fb199611126" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_3bdc6377-f7b3-4e95-b015-286101ac82a6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_8d735daf-da23-4dad-adf8-5fb199611126" xlink:to="loc_us-gaap_AccountsPayableCurrent_3bdc6377-f7b3-4e95-b015-286101ac82a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_10dca591-f897-4aba-8141-85b0a88c6225" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_8d735daf-da23-4dad-adf8-5fb199611126" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_10dca591-f897-4aba-8141-85b0a88c6225" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_AccruedContractedServicesCurrent_20c82f4a-023c-4281-858b-65309d29acdc" xlink:href="qsi-20240930.xsd#qsi_AccruedContractedServicesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_8d735daf-da23-4dad-adf8-5fb199611126" xlink:to="loc_qsi_AccruedContractedServicesCurrent_20c82f4a-023c-4281-858b-65309d29acdc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableAndOtherAccruedLiabilitiesCurrent_59e13875-358b-429b-9e58-3c6cd20bb33b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsPayableAndOtherAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_8d735daf-da23-4dad-adf8-5fb199611126" xlink:to="loc_us-gaap_AccountsPayableAndOtherAccruedLiabilitiesCurrent_59e13875-358b-429b-9e58-3c6cd20bb33b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_ec8a7d5c-eadf-40af-a72f-9ed88750d8eb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_8d735daf-da23-4dad-adf8-5fb199611126" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_ec8a7d5c-eadf-40af-a72f-9ed88750d8eb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_bfe9e71f-69a4-4b2e-8b62-112c4c066f66" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_8d735daf-da23-4dad-adf8-5fb199611126" xlink:to="loc_us-gaap_LiabilitiesCurrent_bfe9e71f-69a4-4b2e-8b62-112c4c066f66" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilitiesNoncurrent_f611026c-5dcf-4fa0-af0b-b67391580f4d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:to="loc_us-gaap_DerivativeLiabilitiesNoncurrent_f611026c-5dcf-4fa0-af0b-b67391580f4d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_0d4be585-9d75-4f51-909e-0f29552952b9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_0d4be585-9d75-4f51-909e-0f29552952b9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_ecf607c6-7a3c-45b9-a85c-cebee1e03334" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_ecf607c6-7a3c-45b9-a85c-cebee1e03334" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_75480576-4aea-4fb2-9e68-d375580cdcf2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:to="loc_us-gaap_Liabilities_75480576-4aea-4fb2-9e68-d375580cdcf2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_446cf801-ce6f-4bfa-80f1-6d90197fec16" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:to="loc_us-gaap_CommitmentsAndContingencies_446cf801-ce6f-4bfa-80f1-6d90197fec16" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_77f97754-5979-4d7e-a74c-49eb47f53669" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:to="loc_us-gaap_StockholdersEquityAbstract_77f97754-5979-4d7e-a74c-49eb47f53669" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_82c34364-d1fc-47e7-b90f-62d8bb29aa12" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_77f97754-5979-4d7e-a74c-49eb47f53669" xlink:to="loc_us-gaap_CommonStockValue_82c34364-d1fc-47e7-b90f-62d8bb29aa12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_4c2b884d-bc0e-4a13-b88e-6d2c429d1d03" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_77f97754-5979-4d7e-a74c-49eb47f53669" xlink:to="loc_us-gaap_AdditionalPaidInCapital_4c2b884d-bc0e-4a13-b88e-6d2c429d1d03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_cdcb7793-a6ec-4ac5-b544-4a08ff156bd2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_77f97754-5979-4d7e-a74c-49eb47f53669" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_cdcb7793-a6ec-4ac5-b544-4a08ff156bd2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_ac23a8fc-fa0e-4ee9-8314-3058255a5269" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_77f97754-5979-4d7e-a74c-49eb47f53669" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_ac23a8fc-fa0e-4ee9-8314-3058255a5269" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_c42de6be-a08d-4234-8aed-5077dfe6cd9d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_77f97754-5979-4d7e-a74c-49eb47f53669" xlink:to="loc_us-gaap_StockholdersEquity_c42de6be-a08d-4234-8aed-5077dfe6cd9d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_10e6c31c-2e7b-4458-b8c7-ded16d5e5c6f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_707fb41b-3ada-4127-af46-b0725bd57b4f" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_10e6c31c-2e7b-4458-b8c7-ded16d5e5c6f" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" xlink:href="qsi-20240930.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_708c0049-73f6-4bf8-8e54-0bb68fb37589" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_a946d9d1-887b-45fb-bcd8-b44072b9fd65" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_708c0049-73f6-4bf8-8e54-0bb68fb37589" xlink:to="loc_us-gaap_StatementTable_a946d9d1-887b-45fb-bcd8-b44072b9fd65" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_465d0ff3-f9fb-45ea-a358-b674a3f1889b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_a946d9d1-887b-45fb-bcd8-b44072b9fd65" xlink:to="loc_us-gaap_StatementClassOfStockAxis_465d0ff3-f9fb-45ea-a358-b674a3f1889b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_465d0ff3-f9fb-45ea-a358-b674a3f1889b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_465d0ff3-f9fb-45ea-a358-b674a3f1889b" xlink:to="loc_us-gaap_ClassOfStockDomain_465d0ff3-f9fb-45ea-a358-b674a3f1889b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_47db087c-3dd4-427a-ac82-e566c6bba53b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_465d0ff3-f9fb-45ea-a358-b674a3f1889b" xlink:to="loc_us-gaap_ClassOfStockDomain_47db087c-3dd4-427a-ac82-e566c6bba53b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_86d92bd0-708d-462d-9c11-eb1f43b9a777" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_47db087c-3dd4-427a-ac82-e566c6bba53b" xlink:to="loc_us-gaap_CommonClassAMember_86d92bd0-708d-462d-9c11-eb1f43b9a777" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_45b6ec64-ada8-4eb4-8aba-2c8a4fc02a3e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_47db087c-3dd4-427a-ac82-e566c6bba53b" xlink:to="loc_us-gaap_CommonClassBMember_45b6ec64-ada8-4eb4-8aba-2c8a4fc02a3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_d06b1938-4b4d-48b9-9d94-43b3aee6bd44" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_708c0049-73f6-4bf8-8e54-0bb68fb37589" xlink:to="loc_us-gaap_AssetsAbstract_d06b1938-4b4d-48b9-9d94-43b3aee6bd44" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_d73eff46-b5d2-44df-9378-90f6dc4cf3c6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_d06b1938-4b4d-48b9-9d94-43b3aee6bd44" xlink:to="loc_us-gaap_AssetsCurrentAbstract_d73eff46-b5d2-44df-9378-90f6dc4cf3c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_6ed2e658-8cdf-4463-b985-3272a7982aad" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_d73eff46-b5d2-44df-9378-90f6dc4cf3c6" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_6ed2e658-8cdf-4463-b985-3272a7982aad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_2cbff162-1c92-4e03-8cd9-90e732cf559d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_708c0049-73f6-4bf8-8e54-0bb68fb37589" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_2cbff162-1c92-4e03-8cd9-90e732cf559d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_6e4f9e6f-f6fc-4bf3-b5a3-b15dfd234f50" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_2cbff162-1c92-4e03-8cd9-90e732cf559d" xlink:to="loc_us-gaap_StockholdersEquityAbstract_6e4f9e6f-f6fc-4bf3-b5a3-b15dfd234f50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_49d92a64-2550-4251-82ad-23d04f0944a2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_6e4f9e6f-f6fc-4bf3-b5a3-b15dfd234f50" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_49d92a64-2550-4251-82ad-23d04f0944a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_340500fa-5d30-464f-884c-7e35adbb2f5b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_6e4f9e6f-f6fc-4bf3-b5a3-b15dfd234f50" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_340500fa-5d30-464f-884c-7e35adbb2f5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_393924cd-1765-449c-a780-6d8392c4c16b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_6e4f9e6f-f6fc-4bf3-b5a3-b15dfd234f50" xlink:to="loc_us-gaap_CommonStockSharesIssued_393924cd-1765-449c-a780-6d8392c4c16b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_3ab3768e-004f-4622-89f3-aaf5388cd0f8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_6e4f9e6f-f6fc-4bf3-b5a3-b15dfd234f50" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_3ab3768e-004f-4622-89f3-aaf5388cd0f8" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSS" xlink:type="simple" xlink:href="qsi-20240930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSS"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_d297efe0-c8f8-465c-9864-f1ca37c36199" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_StatementTable_d297efe0-c8f8-465c-9864-f1ca37c36199" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_fa021b8d-32e8-4d36-a084-34ffc892c71d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_d297efe0-c8f8-465c-9864-f1ca37c36199" xlink:to="loc_srt_ProductOrServiceAxis_fa021b8d-32e8-4d36-a084-34ffc892c71d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_fa021b8d-32e8-4d36-a084-34ffc892c71d_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_fa021b8d-32e8-4d36-a084-34ffc892c71d" xlink:to="loc_srt_ProductsAndServicesDomain_fa021b8d-32e8-4d36-a084-34ffc892c71d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_8f203a0a-ff10-4477-8bd4-bc11de6a4538" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_fa021b8d-32e8-4d36-a084-34ffc892c71d" xlink:to="loc_srt_ProductsAndServicesDomain_8f203a0a-ff10-4477-8bd4-bc11de6a4538" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_5a1dbfbb-6903-45c1-b8b2-acb97b4145ec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8f203a0a-ff10-4477-8bd4-bc11de6a4538" xlink:to="loc_us-gaap_ProductMember_5a1dbfbb-6903-45c1-b8b2-acb97b4145ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_dab4ea10-5c29-481c-94e2-e40c8d54af32" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8f203a0a-ff10-4477-8bd4-bc11de6a4538" xlink:to="loc_us-gaap_ServiceMember_dab4ea10-5c29-481c-94e2-e40c8d54af32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_6fd9c91b-52a8-46b5-8501-c2466961b877" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenuesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_RevenuesAbstract_6fd9c91b-52a8-46b5-8501-c2466961b877" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_a1f82d96-1289-45c2-a6fc-0814e1a98372" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_6fd9c91b-52a8-46b5-8501-c2466961b877" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_a1f82d96-1289-45c2-a6fc-0814e1a98372" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_111518e4-93b0-4ba0-832a-8fdb14bf2338" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_CostOfRevenue_111518e4-93b0-4ba0-832a-8fdb14bf2338" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfit_9e9741a1-09e5-46cc-a33e-fc16f3f93d7e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GrossProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_GrossProfit_9e9741a1-09e5-46cc-a33e-fc16f3f93d7e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_56f920dc-b13a-4865-a725-a5d8dc5ba7fe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_OperatingExpensesAbstract_56f920dc-b13a-4865-a725-a5d8dc5ba7fe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_67e72ad3-7f3c-49b5-98c7-c2409d9880da" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_56f920dc-b13a-4865-a725-a5d8dc5ba7fe" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_67e72ad3-7f3c-49b5-98c7-c2409d9880da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_e8d83ab3-3a66-4a1b-8052-864e3347438a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_56f920dc-b13a-4865-a725-a5d8dc5ba7fe" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_e8d83ab3-3a66-4a1b-8052-864e3347438a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_942c1f5b-6bf5-4f6e-837c-21370dcd662c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_56f920dc-b13a-4865-a725-a5d8dc5ba7fe" xlink:to="loc_us-gaap_OperatingExpenses_942c1f5b-6bf5-4f6e-837c-21370dcd662c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_7d90f8d0-598d-4e1a-8111-c740cbcb5324" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_OperatingIncomeLoss_7d90f8d0-598d-4e1a-8111-c740cbcb5324" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterestAndDividend_85a41ae2-d3a3-49f8-97d1-6f01fc4385f0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InvestmentIncomeInterestAndDividend"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_InvestmentIncomeInterestAndDividend_85a41ae2-d3a3-49f8-97d1-6f01fc4385f0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesTradingUnrealizedGainLoss_e480475b-b11c-40f1-a225-a76d0a60b641" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtSecuritiesTradingUnrealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_DebtSecuritiesTradingUnrealizedGainLoss_e480475b-b11c-40f1-a225-a76d0a60b641" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesTradingRealizedGainLoss_9b973bcd-ebc6-4e9b-91c4-e7c39360b634" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtSecuritiesTradingRealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_DebtSecuritiesTradingRealizedGainLoss_9b973bcd-ebc6-4e9b-91c4-e7c39360b634" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet_2c9a4dd6-a2ba-4bf7-b2e4-9d41f5b49154" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet_2c9a4dd6-a2ba-4bf7-b2e4-9d41f5b49154" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_9bbadaef-a469-4838-8a99-7c618f51f4f3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_9bbadaef-a469-4838-8a99-7c618f51f4f3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_580dea15-6958-4d91-a687-d4ad7b3ae3ff" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_580dea15-6958-4d91-a687-d4ad7b3ae3ff" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_26374fa2-ee4f-493f-9e72-c2ee8332efaa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_26374fa2-ee4f-493f-9e72-c2ee8332efaa" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_6921bc13-12a0-4a0c-892d-97a0fa74b118" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_NetIncomeLoss_6921bc13-12a0-4a0c-892d-97a0fa74b118" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_9bc08e0a-0c55-414f-889d-040ebd0dedd8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_EarningsPerShareBasic_9bc08e0a-0c55-414f-889d-040ebd0dedd8" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_c1ecfde1-6edf-40a5-b8ec-f11c7171a1da" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_EarningsPerShareDiluted_c1ecfde1-6edf-40a5-b8ec-f11c7171a1da" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_20e2a669-5270-4ccc-9b0c-e35a7c3a7896" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_20e2a669-5270-4ccc-9b0c-e35a7c3a7896" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_54b4fc7f-66bf-4286-a0f8-a843438b970e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_54b4fc7f-66bf-4286-a0f8-a843438b970e" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_3a5b5eb4-a2c7-46bc-bd51-52e12ac991b7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_3a5b5eb4-a2c7-46bc-bd51-52e12ac991b7" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_d225a674-5361-43ac-9494-6a0e6c814428" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_3a5b5eb4-a2c7-46bc-bd51-52e12ac991b7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_d225a674-5361-43ac-9494-6a0e6c814428" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_c0ef0897-1700-4627-8a1a-e59a382204a2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_3a5b5eb4-a2c7-46bc-bd51-52e12ac991b7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_c0ef0897-1700-4627-8a1a-e59a382204a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_0d93994c-b177-4437-9472-a8503c1875ca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_3a5b5eb4-a2c7-46bc-bd51-52e12ac991b7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_0d93994c-b177-4437-9472-a8503c1875ca" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_b37658ba-ae76-40d4-88c5-64a8ceb93c96" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_dba850c5-cb11-4db3-8439-980aff6c74bd" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_b37658ba-ae76-40d4-88c5-64a8ceb93c96" xlink:type="arc" order="18"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="qsi-20240930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_b19a97d7-b2ba-4b70-af42-54a6d8ca7bcf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_348c1f9e-aabe-41d5-8496-8b1f3dfee565" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_b19a97d7-b2ba-4b70-af42-54a6d8ca7bcf" xlink:to="loc_us-gaap_StatementTable_348c1f9e-aabe-41d5-8496-8b1f3dfee565" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_48a7826f-b2ea-4a99-9d48-d214d682f23d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_348c1f9e-aabe-41d5-8496-8b1f3dfee565" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_48a7826f-b2ea-4a99-9d48-d214d682f23d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_48a7826f-b2ea-4a99-9d48-d214d682f23d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_48a7826f-b2ea-4a99-9d48-d214d682f23d" xlink:to="loc_us-gaap_EquityComponentDomain_48a7826f-b2ea-4a99-9d48-d214d682f23d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_2557dcdd-b928-4772-b6e8-f626ef1d21ae" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_48a7826f-b2ea-4a99-9d48-d214d682f23d" xlink:to="loc_us-gaap_EquityComponentDomain_2557dcdd-b928-4772-b6e8-f626ef1d21ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_c442592e-badf-40b4-a2ce-2d8cb858ef41" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2557dcdd-b928-4772-b6e8-f626ef1d21ae" xlink:to="loc_us-gaap_CommonStockMember_c442592e-badf-40b4-a2ce-2d8cb858ef41" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_d145266f-ae11-4406-a0d7-9aec074b72c6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2557dcdd-b928-4772-b6e8-f626ef1d21ae" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_d145266f-ae11-4406-a0d7-9aec074b72c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_9ef6cc61-b5c5-4e61-ad17-b9119983f60e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2557dcdd-b928-4772-b6e8-f626ef1d21ae" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_9ef6cc61-b5c5-4e61-ad17-b9119983f60e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_382bed2d-c022-4e5a-aa4c-e53847d8b58c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2557dcdd-b928-4772-b6e8-f626ef1d21ae" xlink:to="loc_us-gaap_RetainedEarningsMember_382bed2d-c022-4e5a-aa4c-e53847d8b58c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_4a3cc46e-a121-4179-93a8-a807024bbe1e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_348c1f9e-aabe-41d5-8496-8b1f3dfee565" xlink:to="loc_us-gaap_StatementClassOfStockAxis_4a3cc46e-a121-4179-93a8-a807024bbe1e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_4a3cc46e-a121-4179-93a8-a807024bbe1e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4a3cc46e-a121-4179-93a8-a807024bbe1e" xlink:to="loc_us-gaap_ClassOfStockDomain_4a3cc46e-a121-4179-93a8-a807024bbe1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f89c8872-392b-4325-b943-8a3068d5388b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4a3cc46e-a121-4179-93a8-a807024bbe1e" xlink:to="loc_us-gaap_ClassOfStockDomain_f89c8872-392b-4325-b943-8a3068d5388b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_4f0cd087-2867-4b31-a146-6e05810a40a1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f89c8872-392b-4325-b943-8a3068d5388b" xlink:to="loc_us-gaap_CommonClassAMember_4f0cd087-2867-4b31-a146-6e05810a40a1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_ca6472f1-aa5b-4e50-a96b-f00553ff7c22" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f89c8872-392b-4325-b943-8a3068d5388b" xlink:to="loc_us-gaap_CommonClassBMember_ca6472f1-aa5b-4e50-a96b-f00553ff7c22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b19a97d7-b2ba-4b70-af42-54a6d8ca7bcf" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_cbff16eb-50c2-4e35-b73c-31ee699bd0e2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_StockholdersEquity_cbff16eb-50c2-4e35-b73c-31ee699bd0e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_6ea51149-03ce-454f-90ea-5f7e114cdb64" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_6ea51149-03ce-454f-90ea-5f7e114cdb64" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_b1769767-283c-416e-a4a8-c4a423d143e7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_b1769767-283c-416e-a4a8-c4a423d143e7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_c86d56c7-4074-4c8d-869b-7da5d365dfca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_c86d56c7-4074-4c8d-869b-7da5d365dfca" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_6d52a836-710e-4ef5-b65c-54fbfd35151c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_6d52a836-710e-4ef5-b65c-54fbfd35151c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_cbce68ef-8e5e-4359-b515-318c262af39d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_cbce68ef-8e5e-4359-b515-318c262af39d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_2dd676d0-38d7-437d-8ec3-fcc841d4dc5d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_2dd676d0-38d7-437d-8ec3-fcc841d4dc5d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_bb24829c-d345-4435-b281-7eff2108aaa8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_bb24829c-d345-4435-b281-7eff2108aaa8" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_fad0561a-caea-49f1-882c-631bb002fd21" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_fad0561a-caea-49f1-882c-631bb002fd21" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_AdjustmentsToAdditionalPaidInCapitalRefundOfIssuanceCosts_9c9d463a-fe12-470d-b8c9-10178764ec35" xlink:href="qsi-20240930.xsd#qsi_AdjustmentsToAdditionalPaidInCapitalRefundOfIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_qsi_AdjustmentsToAdditionalPaidInCapitalRefundOfIssuanceCosts_9c9d463a-fe12-470d-b8c9-10178764ec35" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_a5af159b-5c2b-48ee-9dd2-1867f3d58354" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_a5af159b-5c2b-48ee-9dd2-1867f3d58354" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_1ea3f20f-977c-407a-b7be-33a7cc919729" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a0663e2d-eb1d-456b-93eb-915b80e06c51" xlink:to="loc_us-gaap_NetIncomeLoss_1ea3f20f-977c-407a-b7be-33a7cc919729" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_6b5babc2-fc73-49d9-98d2-c3da9437644f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_a6b72ce2-e443-4896-823e-136c4bdb4219" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#SummaryofSignificantAccountingPoliciesDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_c4954dd7-d2c5-4760-b39f-916fcf506a71" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_b56d5e5d-0e9b-41ed-9e9c-f026c4730e86" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_c4954dd7-d2c5-4760-b39f-916fcf506a71" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_b56d5e5d-0e9b-41ed-9e9c-f026c4730e86" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_dd0bb486-a57e-4438-8998-07b1fba1c056" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_b56d5e5d-0e9b-41ed-9e9c-f026c4730e86" xlink:to="loc_srt_StatementGeographicalAxis_dd0bb486-a57e-4438-8998-07b1fba1c056" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_dd0bb486-a57e-4438-8998-07b1fba1c056_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_dd0bb486-a57e-4438-8998-07b1fba1c056" xlink:to="loc_srt_SegmentGeographicalDomain_dd0bb486-a57e-4438-8998-07b1fba1c056_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_617d7092-e5c4-405b-a067-d8b25338c145" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_dd0bb486-a57e-4438-8998-07b1fba1c056" xlink:to="loc_srt_SegmentGeographicalDomain_617d7092-e5c4-405b-a067-d8b25338c145" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_511dd4c0-c52b-41d9-adcd-5d9f110e0ab5" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_617d7092-e5c4-405b-a067-d8b25338c145" xlink:to="loc_country_US_511dd4c0-c52b-41d9-adcd-5d9f110e0ab5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_788c002a-8dc4-4e39-83b4-abd7c16a39b7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_617d7092-e5c4-405b-a067-d8b25338c145" xlink:to="loc_us-gaap_NonUsMember_788c002a-8dc4-4e39-83b4-abd7c16a39b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_7849cc92-8065-4bb1-9f12-be7fe2b32e2a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_b56d5e5d-0e9b-41ed-9e9c-f026c4730e86" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_7849cc92-8065-4bb1-9f12-be7fe2b32e2a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_c4954dd7-d2c5-4760-b39f-916fcf506a71" xlink:to="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_007c73d1-4d29-4bab-b4bc-d644f26bdcac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_NumberOfReportableSegments_007c73d1-4d29-4bab-b4bc-d644f26bdcac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_StandardProductWarrantyWarrantyPeriod_e94a2319-9c9b-4888-800c-c9612dc8a1a5" xlink:href="qsi-20240930.xsd#qsi_StandardProductWarrantyWarrantyPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_qsi_StandardProductWarrantyWarrantyPeriod_e94a2319-9c9b-4888-800c-c9612dc8a1a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_dbbccfe6-9418-495c-ac63-275184e9eb06" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_dbbccfe6-9418-495c-ac63-275184e9eb06" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_056b0014-618e-493b-8d38-bed167e2c974" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ContractWithCustomerLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_ContractWithCustomerLiability_056b0014-618e-493b-8d38-bed167e2c974" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_49d5f440-1129-4ed9-9a43-7d7aa9ba7a79" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_49d5f440-1129-4ed9-9a43-7d7aa9ba7a79" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_2a2b2ead-2837-4aa4-9333-1ff783aad834" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_2a2b2ead-2837-4aa4-9333-1ff783aad834" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesDetails_1" xlink:type="simple" xlink:href="qsi-20240930.xsd#SummaryofSignificantAccountingPoliciesDetails_1"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_c4954dd7-d2c5-4760-b39f-916fcf506a71" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_b56d5e5d-0e9b-41ed-9e9c-f026c4730e86" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_c4954dd7-d2c5-4760-b39f-916fcf506a71" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_b56d5e5d-0e9b-41ed-9e9c-f026c4730e86" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_dd0bb486-a57e-4438-8998-07b1fba1c056" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_b56d5e5d-0e9b-41ed-9e9c-f026c4730e86" xlink:to="loc_srt_StatementGeographicalAxis_dd0bb486-a57e-4438-8998-07b1fba1c056" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_dd0bb486-a57e-4438-8998-07b1fba1c056_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_dd0bb486-a57e-4438-8998-07b1fba1c056" xlink:to="loc_srt_SegmentGeographicalDomain_dd0bb486-a57e-4438-8998-07b1fba1c056_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_617d7092-e5c4-405b-a067-d8b25338c145" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_dd0bb486-a57e-4438-8998-07b1fba1c056" xlink:to="loc_srt_SegmentGeographicalDomain_617d7092-e5c4-405b-a067-d8b25338c145" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_511dd4c0-c52b-41d9-adcd-5d9f110e0ab5" xlink:href="https://xbrl.sec.gov/country/2024/country-2024.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_617d7092-e5c4-405b-a067-d8b25338c145" xlink:to="loc_country_US_511dd4c0-c52b-41d9-adcd-5d9f110e0ab5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_788c002a-8dc4-4e39-83b4-abd7c16a39b7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_617d7092-e5c4-405b-a067-d8b25338c145" xlink:to="loc_us-gaap_NonUsMember_788c002a-8dc4-4e39-83b4-abd7c16a39b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_c4954dd7-d2c5-4760-b39f-916fcf506a71" xlink:to="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_007c73d1-4d29-4bab-b4bc-d644f26bdcac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_NumberOfReportableSegments_007c73d1-4d29-4bab-b4bc-d644f26bdcac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_StandardProductWarrantyWarrantyPeriod_e94a2319-9c9b-4888-800c-c9612dc8a1a5" xlink:href="qsi-20240930.xsd#qsi_StandardProductWarrantyWarrantyPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_qsi_StandardProductWarrantyWarrantyPeriod_e94a2319-9c9b-4888-800c-c9612dc8a1a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_dbbccfe6-9418-495c-ac63-275184e9eb06" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_dbbccfe6-9418-495c-ac63-275184e9eb06" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_056b0014-618e-493b-8d38-bed167e2c974" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ContractWithCustomerLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_ContractWithCustomerLiability_056b0014-618e-493b-8d38-bed167e2c974" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_49d5f440-1129-4ed9-9a43-7d7aa9ba7a79" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_49d5f440-1129-4ed9-9a43-7d7aa9ba7a79" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_2a2b2ead-2837-4aa4-9333-1ff783aad834" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueAbstract_5f18a382-1153-44c6-bd3d-b6bf708a0389" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_2a2b2ead-2837-4aa4-9333-1ff783aad834" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/InvestmentsinMarketableSecuritiesScheduleofAvailableforSaleSecuritiesReconciliationDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#InvestmentsinMarketableSecuritiesScheduleofAvailableforSaleSecuritiesReconciliationDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/InvestmentsinMarketableSecuritiesScheduleofAvailableforSaleSecuritiesReconciliationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_2209f866-664f-4b7c-be65-12f89ad190f4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_fbfd1d38-97a4-4a13-bd5a-6805cea1611c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_2209f866-664f-4b7c-be65-12f89ad190f4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_fbfd1d38-97a4-4a13-bd5a-6805cea1611c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_54570a58-6a0f-4626-b6ed-23af6f65c892" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_fbfd1d38-97a4-4a13-bd5a-6805cea1611c" xlink:to="loc_us-gaap_FinancialInstrumentAxis_54570a58-6a0f-4626-b6ed-23af6f65c892" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_54570a58-6a0f-4626-b6ed-23af6f65c892_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_54570a58-6a0f-4626-b6ed-23af6f65c892" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_54570a58-6a0f-4626-b6ed-23af6f65c892_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_ac32c912-065c-4e5e-870a-e373d7a24904" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_54570a58-6a0f-4626-b6ed-23af6f65c892" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_ac32c912-065c-4e5e-870a-e373d7a24904" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_1f8e7ff4-47a0-4c51-b5f6-e1255b5ceb3b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_ac32c912-065c-4e5e-870a-e373d7a24904" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_1f8e7ff4-47a0-4c51-b5f6-e1255b5ceb3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_f186b9a9-d5ef-40d7-b1fe-0e6c8fcbf638" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_ac32c912-065c-4e5e-870a-e373d7a24904" xlink:to="loc_us-gaap_CommercialPaperMember_f186b9a9-d5ef-40d7-b1fe-0e6c8fcbf638" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_e0244c23-bd9e-46f7-9922-865cea070ccf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_2209f866-664f-4b7c-be65-12f89ad190f4" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_e0244c23-bd9e-46f7-9922-865cea070ccf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_DebtSecuritiesAvailableForSaleAccumulatedGrossRealizedGainBeforeTax_5457163f-68c0-4557-95d9-6d71ba0cf6da" xlink:href="qsi-20240930.xsd#qsi_DebtSecuritiesAvailableForSaleAccumulatedGrossRealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_2209f866-664f-4b7c-be65-12f89ad190f4" xlink:to="loc_qsi_DebtSecuritiesAvailableForSaleAccumulatedGrossRealizedGainBeforeTax_5457163f-68c0-4557-95d9-6d71ba0cf6da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_ce0c1489-785f-41b3-ba65-bd43baeba09f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_2209f866-664f-4b7c-be65-12f89ad190f4" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_ce0c1489-785f-41b3-ba65-bd43baeba09f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_505752f9-2435-4ab3-9ba3-20f689b49655" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_2209f866-664f-4b7c-be65-12f89ad190f4" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_505752f9-2435-4ab3-9ba3-20f689b49655" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_5df548ab-7630-4a9f-a1b6-340bd89368c8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_2209f866-664f-4b7c-be65-12f89ad190f4" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_5df548ab-7630-4a9f-a1b6-340bd89368c8" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/InvestmentsinMarketableSecuritiesInvestmentsClassifiedbyContractualMaturityDateDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#InvestmentsinMarketableSecuritiesInvestmentsClassifiedbyContractualMaturityDateDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/InvestmentsinMarketableSecuritiesInvestmentsClassifiedbyContractualMaturityDateDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_0c14d215-8aca-4d59-b059-bba92db21371" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_3c7e6be9-4b46-4820-97db-14055154ff4e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_0c14d215-8aca-4d59-b059-bba92db21371" xlink:to="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_3c7e6be9-4b46-4820-97db-14055154ff4e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_90ef11a9-e413-48e2-a5da-d95f5482822b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_3c7e6be9-4b46-4820-97db-14055154ff4e" xlink:to="loc_us-gaap_FinancialInstrumentAxis_90ef11a9-e413-48e2-a5da-d95f5482822b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_90ef11a9-e413-48e2-a5da-d95f5482822b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_90ef11a9-e413-48e2-a5da-d95f5482822b" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_90ef11a9-e413-48e2-a5da-d95f5482822b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1d9ffe63-8e9d-46f9-9ba4-b5e1d3726f18" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_90ef11a9-e413-48e2-a5da-d95f5482822b" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1d9ffe63-8e9d-46f9-9ba4-b5e1d3726f18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_d224f4d0-3b36-48fe-997d-6b39fd526d94" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1d9ffe63-8e9d-46f9-9ba4-b5e1d3726f18" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_d224f4d0-3b36-48fe-997d-6b39fd526d94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_13403d85-1454-4690-bbf3-b7b2552f2e73" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1d9ffe63-8e9d-46f9-9ba4-b5e1d3726f18" xlink:to="loc_us-gaap_CommercialPaperMember_13403d85-1454-4690-bbf3-b7b2552f2e73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_40d240f9-b94b-417e-992f-13b145831adf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_0c14d215-8aca-4d59-b059-bba92db21371" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_40d240f9-b94b-417e-992f-13b145831adf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_1d4cfe78-e850-40c5-a910-455d831b30aa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_0c14d215-8aca-4d59-b059-bba92db21371" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_1d4cfe78-e850-40c5-a910-455d831b30aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost_c9315e54-7ad9-49fd-9d73-e85aaba54d03" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_0c14d215-8aca-4d59-b059-bba92db21371" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost_c9315e54-7ad9-49fd-9d73-e85aaba54d03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis_a0907e0f-2bdb-4273-9781-1a03c79c586c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_0c14d215-8aca-4d59-b059-bba92db21371" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis_a0907e0f-2bdb-4273-9781-1a03c79c586c" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/FairValueofFinancialInstrumentsNarrativeDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#FairValueofFinancialInstrumentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/FairValueofFinancialInstrumentsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_813ed03f-2541-4e69-8a66-35e764ca260f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_83608fd2-c35d-4c72-ad2d-25e24b69e622" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_813ed03f-2541-4e69-8a66-35e764ca260f" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_83608fd2-c35d-4c72-ad2d-25e24b69e622" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_edc2f1c6-b191-4229-a30f-2b27b6c69a21" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_83608fd2-c35d-4c72-ad2d-25e24b69e622" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_edc2f1c6-b191-4229-a30f-2b27b6c69a21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_edc2f1c6-b191-4229-a30f-2b27b6c69a21_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_edc2f1c6-b191-4229-a30f-2b27b6c69a21" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_edc2f1c6-b191-4229-a30f-2b27b6c69a21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_9245fb38-8dbf-4b88-9dec-3122defd4c8b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_edc2f1c6-b191-4229-a30f-2b27b6c69a21" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_9245fb38-8dbf-4b88-9dec-3122defd4c8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_PrivatePlacementWarrantsMember_098e398f-2f7f-4482-9ff5-f1130e1a3436" xlink:href="qsi-20240930.xsd#qsi_PrivatePlacementWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_9245fb38-8dbf-4b88-9dec-3122defd4c8b" xlink:to="loc_qsi_PrivatePlacementWarrantsMember_098e398f-2f7f-4482-9ff5-f1130e1a3436" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_6f62684a-386b-48dc-a49f-9ab117a23b4d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_83608fd2-c35d-4c72-ad2d-25e24b69e622" xlink:to="loc_us-gaap_FinancialInstrumentAxis_6f62684a-386b-48dc-a49f-9ab117a23b4d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6f62684a-386b-48dc-a49f-9ab117a23b4d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_6f62684a-386b-48dc-a49f-9ab117a23b4d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6f62684a-386b-48dc-a49f-9ab117a23b4d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_2449847a-2b36-4629-8752-d2051b8e3b9e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_6f62684a-386b-48dc-a49f-9ab117a23b4d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_2449847a-2b36-4629-8752-d2051b8e3b9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_dae37db0-429d-431d-89a9-ce0e4febb7fa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_2449847a-2b36-4629-8752-d2051b8e3b9e" xlink:to="loc_us-gaap_WarrantMember_dae37db0-429d-431d-89a9-ce0e4febb7fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_3f217c28-9177-4efd-9e0b-b8ff1eb170a1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_83608fd2-c35d-4c72-ad2d-25e24b69e622" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_3f217c28-9177-4efd-9e0b-b8ff1eb170a1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_3f217c28-9177-4efd-9e0b-b8ff1eb170a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_3f217c28-9177-4efd-9e0b-b8ff1eb170a1" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_3f217c28-9177-4efd-9e0b-b8ff1eb170a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_510097a8-5ff0-4364-8169-da0ed4fb1ed4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_3f217c28-9177-4efd-9e0b-b8ff1eb170a1" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_510097a8-5ff0-4364-8169-da0ed4fb1ed4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_46e3cb44-f824-4e74-b40a-8537903d4a45" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputPriceVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_510097a8-5ff0-4364-8169-da0ed4fb1ed4" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_46e3cb44-f824-4e74-b40a-8537903d4a45" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_d7b8a8b4-09bc-42e8-b42d-77f783bf9ebc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_510097a8-5ff0-4364-8169-da0ed4fb1ed4" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_d7b8a8b4-09bc-42e8-b42d-77f783bf9ebc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExercisePriceMember_7e33b8d1-dc4e-40b9-a4db-d506a3c9b383" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputExercisePriceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_510097a8-5ff0-4364-8169-da0ed4fb1ed4" xlink:to="loc_us-gaap_MeasurementInputExercisePriceMember_7e33b8d1-dc4e-40b9-a4db-d506a3c9b383" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputSharePriceMember_417dfe63-b551-4b6b-9982-5b7ff743c297" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputSharePriceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_510097a8-5ff0-4364-8169-da0ed4fb1ed4" xlink:to="loc_us-gaap_MeasurementInputSharePriceMember_417dfe63-b551-4b6b-9982-5b7ff743c297" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_a22bed71-da1c-4a63-86a3-d1b4c15e8613" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputExpectedTermMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_510097a8-5ff0-4364-8169-da0ed4fb1ed4" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_a22bed71-da1c-4a63-86a3-d1b4c15e8613" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_a8589de7-e4ea-410a-b2e9-9546981d72a3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_813ed03f-2541-4e69-8a66-35e764ca260f" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_a8589de7-e4ea-410a-b2e9-9546981d72a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_3b112f10-1a5b-4131-93e1-f55d99f60183" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_813ed03f-2541-4e69-8a66-35e764ca260f" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_3b112f10-1a5b-4131-93e1-f55d99f60183" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/FairValueofFinancialInstrumentsAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#FairValueofFinancialInstrumentsAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/FairValueofFinancialInstrumentsAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1b7ed926-fd0c-4b02-83fe-b2e776f03edf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_ea5ec801-231c-475e-addb-89149c4ea502" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1b7ed926-fd0c-4b02-83fe-b2e776f03edf" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_ea5ec801-231c-475e-addb-89149c4ea502" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_53ff5d02-0936-4fe6-b927-5e74de459fc0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_ea5ec801-231c-475e-addb-89149c4ea502" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_53ff5d02-0936-4fe6-b927-5e74de459fc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_53ff5d02-0936-4fe6-b927-5e74de459fc0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_53ff5d02-0936-4fe6-b927-5e74de459fc0" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_53ff5d02-0936-4fe6-b927-5e74de459fc0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_111f7fff-1160-46f2-a9b5-73a3fadc29ca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_53ff5d02-0936-4fe6-b927-5e74de459fc0" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_111f7fff-1160-46f2-a9b5-73a3fadc29ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_4405f833-403d-4bce-8938-8ff498af8f6c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_111f7fff-1160-46f2-a9b5-73a3fadc29ca" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_4405f833-403d-4bce-8938-8ff498af8f6c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a0338eb1-1310-45c9-802b-adfe78b07e63" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_ea5ec801-231c-475e-addb-89149c4ea502" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a0338eb1-1310-45c9-802b-adfe78b07e63" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a0338eb1-1310-45c9-802b-adfe78b07e63_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a0338eb1-1310-45c9-802b-adfe78b07e63" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a0338eb1-1310-45c9-802b-adfe78b07e63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3a8774f9-f07c-4a8a-9262-92e9ade7ab5f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a0338eb1-1310-45c9-802b-adfe78b07e63" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3a8774f9-f07c-4a8a-9262-92e9ade7ab5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_f7f226cc-5319-4be2-bf80-b0ac30cbd8e3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3a8774f9-f07c-4a8a-9262-92e9ade7ab5f" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_f7f226cc-5319-4be2-bf80-b0ac30cbd8e3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_898c4294-999d-40d9-8dd9-ad4e89546b12" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3a8774f9-f07c-4a8a-9262-92e9ade7ab5f" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_898c4294-999d-40d9-8dd9-ad4e89546b12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_64b190ff-974e-4e04-ab27-50f78209a9a1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3a8774f9-f07c-4a8a-9262-92e9ade7ab5f" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_64b190ff-974e-4e04-ab27-50f78209a9a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_d1137d23-13ea-408f-aa0d-9d01f89d3d59" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_ea5ec801-231c-475e-addb-89149c4ea502" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_d1137d23-13ea-408f-aa0d-9d01f89d3d59" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_d1137d23-13ea-408f-aa0d-9d01f89d3d59_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_d1137d23-13ea-408f-aa0d-9d01f89d3d59" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_d1137d23-13ea-408f-aa0d-9d01f89d3d59_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_c49868b8-4ba0-4a45-8cc2-f3bc7bf99650" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_d1137d23-13ea-408f-aa0d-9d01f89d3d59" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_c49868b8-4ba0-4a45-8cc2-f3bc7bf99650" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_RedeemableWarrantsMember_88a7d823-c250-4b07-b6be-1d165921646b" xlink:href="qsi-20240930.xsd#qsi_RedeemableWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_c49868b8-4ba0-4a45-8cc2-f3bc7bf99650" xlink:to="loc_qsi_RedeemableWarrantsMember_88a7d823-c250-4b07-b6be-1d165921646b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_PrivatePlacementWarrantsMember_1fcb7e46-cf80-42af-9f83-100a19fb21ba" xlink:href="qsi-20240930.xsd#qsi_PrivatePlacementWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_c49868b8-4ba0-4a45-8cc2-f3bc7bf99650" xlink:to="loc_qsi_PrivatePlacementWarrantsMember_1fcb7e46-cf80-42af-9f83-100a19fb21ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_4d2b38b2-8474-4537-9ce1-7daf78c00bec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_ea5ec801-231c-475e-addb-89149c4ea502" xlink:to="loc_us-gaap_FinancialInstrumentAxis_4d2b38b2-8474-4537-9ce1-7daf78c00bec" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4d2b38b2-8474-4537-9ce1-7daf78c00bec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_4d2b38b2-8474-4537-9ce1-7daf78c00bec" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_4d2b38b2-8474-4537-9ce1-7daf78c00bec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_f7fcb97c-2bb2-4fa6-8080-34f2132034d1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_4d2b38b2-8474-4537-9ce1-7daf78c00bec" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_f7fcb97c-2bb2-4fa6-8080-34f2132034d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_6785794e-2129-4cc4-9c90-2745818b1202" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_f7fcb97c-2bb2-4fa6-8080-34f2132034d1" xlink:to="loc_us-gaap_MoneyMarketFundsMember_6785794e-2129-4cc4-9c90-2745818b1202" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_94496eb1-b163-4268-8f38-0c05cf9a5d7a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_f7fcb97c-2bb2-4fa6-8080-34f2132034d1" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_94496eb1-b163-4268-8f38-0c05cf9a5d7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_05236419-54d1-4a74-8040-8b4bb3e2b941" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_f7fcb97c-2bb2-4fa6-8080-34f2132034d1" xlink:to="loc_us-gaap_CommercialPaperMember_05236419-54d1-4a74-8040-8b4bb3e2b941" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_8c48bc7e-becc-4035-9276-05f459010786" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1b7ed926-fd0c-4b02-83fe-b2e776f03edf" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_8c48bc7e-becc-4035-9276-05f459010786" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradingSecuritiesDebt_b5d2c884-2971-4530-889b-87e1beb6f347" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TradingSecuritiesDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1b7ed926-fd0c-4b02-83fe-b2e776f03edf" xlink:to="loc_us-gaap_TradingSecuritiesDebt_b5d2c884-2971-4530-889b-87e1beb6f347" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_bb5ab7db-bc14-41ac-a6e4-f3e063024a86" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1b7ed926-fd0c-4b02-83fe-b2e776f03edf" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_bb5ab7db-bc14-41ac-a6e4-f3e063024a86" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilitiesNoncurrent_1345623d-30c7-4ec9-bae4-673e73b83750" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1b7ed926-fd0c-4b02-83fe-b2e776f03edf" xlink:to="loc_us-gaap_DerivativeLiabilitiesNoncurrent_1345623d-30c7-4ec9-bae4-673e73b83750" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_02ab6473-55ae-4728-8afb-f02e39a62661" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1b7ed926-fd0c-4b02-83fe-b2e776f03edf" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_02ab6473-55ae-4728-8afb-f02e39a62661" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/InventoryNarrativeDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#InventoryNarrativeDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/InventoryNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryLineItems_3826d349-ad7e-4a6e-9942-fc26550b62ff" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InventoryLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryCurrentTable_e9fffefb-bc4f-4548-b8f8-b62eb694f729" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InventoryCurrentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InventoryLineItems_3826d349-ad7e-4a6e-9942-fc26550b62ff" xlink:to="loc_us-gaap_InventoryCurrentTable_e9fffefb-bc4f-4548-b8f8-b62eb694f729" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_26282e04-cc12-4651-9baf-8726a678011e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InventoryCurrentTable_e9fffefb-bc4f-4548-b8f8-b62eb694f729" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_26282e04-cc12-4651-9baf-8726a678011e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_26282e04-cc12-4651-9baf-8726a678011e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_26282e04-cc12-4651-9baf-8726a678011e" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_26282e04-cc12-4651-9baf-8726a678011e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_22b331a6-a6ac-4eba-9e2b-7b3092588794" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_26282e04-cc12-4651-9baf-8726a678011e" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_22b331a6-a6ac-4eba-9e2b-7b3092588794" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_cb1610ef-8fd2-4a8a-8f02-1324fb72edc0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_22b331a6-a6ac-4eba-9e2b-7b3092588794" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_cb1610ef-8fd2-4a8a-8f02-1324fb72edc0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesChargedToCostAndExpense_b2f1742a-0b5b-42fb-8463-78578f53034d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ValuationAllowancesAndReservesChargedToCostAndExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_3826d349-ad7e-4a6e-9942-fc26550b62ff" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesChargedToCostAndExpense_b2f1742a-0b5b-42fb-8463-78578f53034d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/PropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#PropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/PropertyandEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7fde62b0-88bd-41b2-8405-a750f201b264" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3d991d4e-b800-43eb-a9f2-cb2094657fbd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7fde62b0-88bd-41b2-8405-a750f201b264" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3d991d4e-b800-43eb-a9f2-cb2094657fbd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_82e96250-7cd3-452a-a861-d0e8e40ed54b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3d991d4e-b800-43eb-a9f2-cb2094657fbd" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_82e96250-7cd3-452a-a861-d0e8e40ed54b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_82e96250-7cd3-452a-a861-d0e8e40ed54b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_82e96250-7cd3-452a-a861-d0e8e40ed54b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_82e96250-7cd3-452a-a861-d0e8e40ed54b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2daad5bb-366c-4d16-ae24-c6aa5419ec9f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_82e96250-7cd3-452a-a861-d0e8e40ed54b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2daad5bb-366c-4d16-ae24-c6aa5419ec9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquipmentMember_265a696f-0869-4d8e-9b98-5b1324a18875" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2daad5bb-366c-4d16-ae24-c6aa5419ec9f" xlink:to="loc_us-gaap_EquipmentMember_265a696f-0869-4d8e-9b98-5b1324a18875" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_e9be5ade-339c-42b1-88f5-04d641a71e65" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2daad5bb-366c-4d16-ae24-c6aa5419ec9f" xlink:to="loc_us-gaap_ComputerEquipmentMember_e9be5ade-339c-42b1-88f5-04d641a71e65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_c158b0cc-5305-4c66-8ea0-aea29cb64133" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2daad5bb-366c-4d16-ae24-c6aa5419ec9f" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_c158b0cc-5305-4c66-8ea0-aea29cb64133" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_928ec07a-65d1-4394-bea1-b14d99dc7794" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2daad5bb-366c-4d16-ae24-c6aa5419ec9f" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_928ec07a-65d1-4394-bea1-b14d99dc7794" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_1e1b6a91-e831-42ae-adc1-c4080d3f8fdb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2daad5bb-366c-4d16-ae24-c6aa5419ec9f" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_1e1b6a91-e831-42ae-adc1-c4080d3f8fdb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_e2826236-f173-4ba5-8804-882263119995" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2daad5bb-366c-4d16-ae24-c6aa5419ec9f" xlink:to="loc_us-gaap_ConstructionInProgressMember_e2826236-f173-4ba5-8804-882263119995" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNetAbstract_efdb9272-4df3-4e32-a871-7d3079aa1ae1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7fde62b0-88bd-41b2-8405-a750f201b264" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNetAbstract_efdb9272-4df3-4e32-a871-7d3079aa1ae1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_38d9bcf4-7689-4ab6-85ab-2fc067d629c2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetAbstract_efdb9272-4df3-4e32-a871-7d3079aa1ae1" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_38d9bcf4-7689-4ab6-85ab-2fc067d629c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_32b8915b-c6b9-4209-a56a-e525e370359e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetAbstract_efdb9272-4df3-4e32-a871-7d3079aa1ae1" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_32b8915b-c6b9-4209-a56a-e525e370359e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_5598303f-999c-4b0d-8190-1089073c1f48" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetAbstract_efdb9272-4df3-4e32-a871-7d3079aa1ae1" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_5598303f-999c-4b0d-8190-1089073c1f48" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_3c6420f5-26b2-435d-b23a-90c77bcefb41" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7fde62b0-88bd-41b2-8405-a750f201b264" xlink:to="loc_us-gaap_DepreciationAndAmortization_3c6420f5-26b2-435d-b23a-90c77bcefb41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_f7fed686-008e-4b3d-a31d-087bee7d68a3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_7fde62b0-88bd-41b2-8405-a750f201b264" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_f7fed686-008e-4b3d-a31d-087bee7d68a3" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/LeasesNarrativeDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#LeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/LeasesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_fe60489b-1482-4bea-a262-07853493d301" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_9c1149de-5f6c-499e-923a-8fd42f4f2190" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_fe60489b-1482-4bea-a262-07853493d301" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_9c1149de-5f6c-499e-923a-8fd42f4f2190" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c4061a8e-a798-4ebc-a7cc-43d9d67fb560" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_9c1149de-5f6c-499e-923a-8fd42f4f2190" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c4061a8e-a798-4ebc-a7cc-43d9d67fb560" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_c4061a8e-a798-4ebc-a7cc-43d9d67fb560_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c4061a8e-a798-4ebc-a7cc-43d9d67fb560" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_c4061a8e-a798-4ebc-a7cc-43d9d67fb560_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f9901bcc-8352-4031-be44-00305dfa07a0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c4061a8e-a798-4ebc-a7cc-43d9d67fb560" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f9901bcc-8352-4031-be44-00305dfa07a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_SpaceLocatedInNewHavenConnecticutMember_a592347c-ba50-4d04-ab23-7f0611360f28" xlink:href="qsi-20240930.xsd#qsi_SpaceLocatedInNewHavenConnecticutMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f9901bcc-8352-4031-be44-00305dfa07a0" xlink:to="loc_qsi_SpaceLocatedInNewHavenConnecticutMember_a592347c-ba50-4d04-ab23-7f0611360f28" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseDescriptionAbstract_910dad7e-036f-49b2-b76a-707f70f7f942" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LesseeOperatingLeaseDescriptionAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_fe60489b-1482-4bea-a262-07853493d301" xlink:to="loc_us-gaap_LesseeOperatingLeaseDescriptionAbstract_910dad7e-036f-49b2-b76a-707f70f7f942" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_5f8a1d93-b574-43fb-85b4-b3cf3eaa5172" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeOperatingLeaseDescriptionAbstract_910dad7e-036f-49b2-b76a-707f70f7f942" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_5f8a1d93-b574-43fb-85b4-b3cf3eaa5172" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_8ae3d51e-b154-40fa-8dc8-efe4612fd07a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AreaOfRealEstateProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeOperatingLeaseDescriptionAbstract_910dad7e-036f-49b2-b76a-707f70f7f942" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_8ae3d51e-b154-40fa-8dc8-efe4612fd07a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_LeaseIncentives_5edda79f-0cf4-409d-af0b-9a088605e0a6" xlink:href="qsi-20240930.xsd#qsi_LeaseIncentives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeOperatingLeaseDescriptionAbstract_910dad7e-036f-49b2-b76a-707f70f7f942" xlink:to="loc_qsi_LeaseIncentives_5edda79f-0cf4-409d-af0b-9a088605e0a6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_OperatingLeaseRightOfUseAssetIncrease_f99046e3-202a-4a3b-a90b-079b8450b319" xlink:href="qsi-20240930.xsd#qsi_OperatingLeaseRightOfUseAssetIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeOperatingLeaseDescriptionAbstract_910dad7e-036f-49b2-b76a-707f70f7f942" xlink:to="loc_qsi_OperatingLeaseRightOfUseAssetIncrease_f99046e3-202a-4a3b-a90b-079b8450b319" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsGross_0d20e056-a9dd-404c-95ad-40f9cfb8cee3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LeaseholdImprovementsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeOperatingLeaseDescriptionAbstract_910dad7e-036f-49b2-b76a-707f70f7f942" xlink:to="loc_us-gaap_LeaseholdImprovementsGross_0d20e056-a9dd-404c-95ad-40f9cfb8cee3" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/StockbasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#StockbasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/StockbasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_4493d36a-000a-48c9-ba60-219d6888ce3c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_4493d36a-000a-48c9-ba60-219d6888ce3c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_aa3adde7-384b-4cbe-be02-b4009391c29f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_4493d36a-000a-48c9-ba60-219d6888ce3c" xlink:to="loc_us-gaap_PlanNameAxis_aa3adde7-384b-4cbe-be02-b4009391c29f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_aa3adde7-384b-4cbe-be02-b4009391c29f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_aa3adde7-384b-4cbe-be02-b4009391c29f" xlink:to="loc_us-gaap_PlanNameDomain_aa3adde7-384b-4cbe-be02-b4009391c29f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_5459b558-f2e2-49c7-ba09-d2e8130bd0e4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_aa3adde7-384b-4cbe-be02-b4009391c29f" xlink:to="loc_us-gaap_PlanNameDomain_5459b558-f2e2-49c7-ba09-d2e8130bd0e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_QuantumSiIncorporated2021EquityIncentivePlanMember_a5bf29b8-3151-4abe-a20d-8bbb81268097" xlink:href="qsi-20240930.xsd#qsi_QuantumSiIncorporated2021EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_5459b558-f2e2-49c7-ba09-d2e8130bd0e4" xlink:to="loc_qsi_QuantumSiIncorporated2021EquityIncentivePlanMember_a5bf29b8-3151-4abe-a20d-8bbb81268097" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_InducementEquityIncentivePlan2023Member_3a275964-cd3b-4690-adc6-527369bb0e88" xlink:href="qsi-20240930.xsd#qsi_InducementEquityIncentivePlan2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_5459b558-f2e2-49c7-ba09-d2e8130bd0e4" xlink:to="loc_qsi_InducementEquityIncentivePlan2023Member_3a275964-cd3b-4690-adc6-527369bb0e88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_159e8f1b-79d1-489b-82c5-36be81c6143b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_4493d36a-000a-48c9-ba60-219d6888ce3c" xlink:to="loc_us-gaap_AwardTypeAxis_159e8f1b-79d1-489b-82c5-36be81c6143b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_159e8f1b-79d1-489b-82c5-36be81c6143b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_159e8f1b-79d1-489b-82c5-36be81c6143b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_159e8f1b-79d1-489b-82c5-36be81c6143b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_493287e0-3dd8-4f00-bde6-53825bb51963" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_159e8f1b-79d1-489b-82c5-36be81c6143b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_493287e0-3dd8-4f00-bde6-53825bb51963" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_cce8d15e-aa62-4f79-b8ff-f73364836dfc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_493287e0-3dd8-4f00-bde6-53825bb51963" xlink:to="loc_us-gaap_EmployeeStockOptionMember_cce8d15e-aa62-4f79-b8ff-f73364836dfc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_c0e1ef72-2ed7-4e65-bf22-069f488ab4ba" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_493287e0-3dd8-4f00-bde6-53825bb51963" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_c0e1ef72-2ed7-4e65-bf22-069f488ab4ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_51577775-7a4f-45a9-be8e-abe4cc118f56" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_4493d36a-000a-48c9-ba60-219d6888ce3c" xlink:to="loc_srt_StatementScenarioAxis_51577775-7a4f-45a9-be8e-abe4cc118f56" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_51577775-7a4f-45a9-be8e-abe4cc118f56_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_51577775-7a4f-45a9-be8e-abe4cc118f56" xlink:to="loc_srt_ScenarioUnspecifiedDomain_51577775-7a4f-45a9-be8e-abe4cc118f56_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_5cc4d0c3-c542-48ed-9e38-27535b624930" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_51577775-7a4f-45a9-be8e-abe4cc118f56" xlink:to="loc_srt_ScenarioUnspecifiedDomain_5cc4d0c3-c542-48ed-9e38-27535b624930" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScenarioPlanMember_10a72909-de02-4538-8f45-8c35788f14ad" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScenarioPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_5cc4d0c3-c542-48ed-9e38-27535b624930" xlink:to="loc_us-gaap_ScenarioPlanMember_10a72909-de02-4538-8f45-8c35788f14ad" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_85ddc189-09ff-4c91-a256-975d18bdcd87" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_4493d36a-000a-48c9-ba60-219d6888ce3c" xlink:to="loc_us-gaap_AwardDateAxis_85ddc189-09ff-4c91-a256-975d18bdcd87" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_85ddc189-09ff-4c91-a256-975d18bdcd87_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_85ddc189-09ff-4c91-a256-975d18bdcd87" xlink:to="loc_us-gaap_AwardDateDomain_85ddc189-09ff-4c91-a256-975d18bdcd87_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_eaeb1390-1404-47b2-a64f-fd35b8609d1c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_85ddc189-09ff-4c91-a256-975d18bdcd87" xlink:to="loc_us-gaap_AwardDateDomain_eaeb1390-1404-47b2-a64f-fd35b8609d1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_AwardedNovember92022Member_4a24380c-8f78-47ae-930b-325b0ffbbbe4" xlink:href="qsi-20240930.xsd#qsi_AwardedNovember92022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_eaeb1390-1404-47b2-a64f-fd35b8609d1c" xlink:to="loc_qsi_AwardedNovember92022Member_4a24380c-8f78-47ae-930b-325b0ffbbbe4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_bb5d8ffa-cada-44b9-903e-d27f66744ddd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_bb5d8ffa-cada-44b9-903e-d27f66744ddd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_98d60663-65bd-4326-8b51-0bbe6689c717" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_98d60663-65bd-4326-8b51-0bbe6689c717" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_CommonStockAdditionalCapitalSharesReservedForFutureIssuance_6c8f9b75-4ce4-431d-abd3-7157d62e4e6f" xlink:href="qsi-20240930.xsd#qsi_CommonStockAdditionalCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_qsi_CommonStockAdditionalCapitalSharesReservedForFutureIssuance_6c8f9b75-4ce4-431d-abd3-7157d62e4e6f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockOptionPlanExpense_8ccb6ca8-1605-407d-a7af-04cbfd7f4dbe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockOptionPlanExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_us-gaap_StockOptionPlanExpense_8ccb6ca8-1605-407d-a7af-04cbfd7f4dbe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_a957d0e3-8ee9-4674-bc3a-c176f5f8dc0f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_a957d0e3-8ee9-4674-bc3a-c176f5f8dc0f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_ShareBasedPaymentArrangementPlanModificationToBeRecognized_0fd0209f-8c51-4851-8d4c-9b189f5ab5b1" xlink:href="qsi-20240930.xsd#qsi_ShareBasedPaymentArrangementPlanModificationToBeRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_qsi_ShareBasedPaymentArrangementPlanModificationToBeRecognized_0fd0209f-8c51-4851-8d4c-9b189f5ab5b1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost_751da889-045a-4bad-8414-95e7f7604958" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost_751da889-045a-4bad-8414-95e7f7604958" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_RestrictedStockExpenseCredit_7e22b062-908d-44cd-8e89-178fdebad8a6" xlink:href="qsi-20240930.xsd#qsi_RestrictedStockExpenseCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b320b371-7851-4dde-87a7-90bc49a13e83" xlink:to="loc_qsi_RestrictedStockExpenseCredit_7e22b062-908d-44cd-8e89-178fdebad8a6" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/StockbasedCompensationFairValueofEachStockOptionAwardDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#StockbasedCompensationFairValueofEachStockOptionAwardDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/StockbasedCompensationFairValueofEachStockOptionAwardDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a1d35a7e-839e-4f36-8392-8e8efebc5887" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d8c21837-0ac5-411b-85a8-142893400676" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a1d35a7e-839e-4f36-8392-8e8efebc5887" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d8c21837-0ac5-411b-85a8-142893400676" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_cde165a0-3218-4064-83c7-e33e700c65fc" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d8c21837-0ac5-411b-85a8-142893400676" xlink:to="loc_srt_RangeAxis_cde165a0-3218-4064-83c7-e33e700c65fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_cde165a0-3218-4064-83c7-e33e700c65fc_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_cde165a0-3218-4064-83c7-e33e700c65fc" xlink:to="loc_srt_RangeMember_cde165a0-3218-4064-83c7-e33e700c65fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_eb9741da-8553-4cf6-aca6-2811c7abe0d6" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_cde165a0-3218-4064-83c7-e33e700c65fc" xlink:to="loc_srt_RangeMember_eb9741da-8553-4cf6-aca6-2811c7abe0d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_a9411074-998c-42b3-bf41-8966ea7e0c7b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_eb9741da-8553-4cf6-aca6-2811c7abe0d6" xlink:to="loc_srt_MinimumMember_a9411074-998c-42b3-bf41-8966ea7e0c7b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_0471b8bc-8d86-46ae-b6c1-9bd1d5b4da3a" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_eb9741da-8553-4cf6-aca6-2811c7abe0d6" xlink:to="loc_srt_MaximumMember_0471b8bc-8d86-46ae-b6c1-9bd1d5b4da3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusAxis_1e06d832-63fa-446a-9094-9431cd8fba73" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GranteeStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d8c21837-0ac5-411b-85a8-142893400676" xlink:to="loc_us-gaap_GranteeStatusAxis_1e06d832-63fa-446a-9094-9431cd8fba73" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_1e06d832-63fa-446a-9094-9431cd8fba73_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GranteeStatusAxis_1e06d832-63fa-446a-9094-9431cd8fba73" xlink:to="loc_us-gaap_GranteeStatusDomain_1e06d832-63fa-446a-9094-9431cd8fba73_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_2557295d-78bd-4913-9cad-4e5b9bd55c95" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GranteeStatusAxis_1e06d832-63fa-446a-9094-9431cd8fba73" xlink:to="loc_us-gaap_GranteeStatusDomain_2557295d-78bd-4913-9cad-4e5b9bd55c95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_dd7ce979-2a00-4060-8571-e6e05ef30810" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedPaymentArrangementEmployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_2557295d-78bd-4913-9cad-4e5b9bd55c95" xlink:to="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_dd7ce979-2a00-4060-8571-e6e05ef30810" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_59ac7c29-8ed0-47c4-8fae-f618f3ee794e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a1d35a7e-839e-4f36-8392-8e8efebc5887" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_59ac7c29-8ed0-47c4-8fae-f618f3ee794e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_e09b1674-d611-4518-bea1-72c3f383f86c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a1d35a7e-839e-4f36-8392-8e8efebc5887" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_e09b1674-d611-4518-bea1-72c3f383f86c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_169ca7bf-8790-4476-bc93-3957c0343c5f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a1d35a7e-839e-4f36-8392-8e8efebc5887" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_169ca7bf-8790-4476-bc93-3957c0343c5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_cb1ac8d2-07ec-4252-bde2-0147a0052c3b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a1d35a7e-839e-4f36-8392-8e8efebc5887" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_cb1ac8d2-07ec-4252-bde2-0147a0052c3b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_b3c1ef0c-ca01-404c-a23e-4959cb2c398d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a1d35a7e-839e-4f36-8392-8e8efebc5887" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_b3c1ef0c-ca01-404c-a23e-4959cb2c398d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/StockbasedCompensationStockOptionsDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#StockbasedCompensationStockOptionsDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/StockbasedCompensationStockOptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_630bb12b-58f8-48aa-85ba-0ea4d2ddfbe8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_630bb12b-58f8-48aa-85ba-0ea4d2ddfbe8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_98a8361d-1038-4ef8-992c-0dfd1ddb5454" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_630bb12b-58f8-48aa-85ba-0ea4d2ddfbe8" xlink:to="loc_us-gaap_AwardTypeAxis_98a8361d-1038-4ef8-992c-0dfd1ddb5454" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_98a8361d-1038-4ef8-992c-0dfd1ddb5454_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_98a8361d-1038-4ef8-992c-0dfd1ddb5454" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_98a8361d-1038-4ef8-992c-0dfd1ddb5454_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77426bbe-97bc-4c6c-be11-424dc6863d6a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_98a8361d-1038-4ef8-992c-0dfd1ddb5454" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77426bbe-97bc-4c6c-be11-424dc6863d6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_388b9919-4365-4661-9966-7086170f7f2c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77426bbe-97bc-4c6c-be11-424dc6863d6a" xlink:to="loc_us-gaap_EmployeeStockOptionMember_388b9919-4365-4661-9966-7086170f7f2c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d7db7cbd-cde9-4637-96e9-faac9cec5673" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d7db7cbd-cde9-4637-96e9-faac9cec5673" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_e9b6cc0c-cc84-412f-ab03-a3b2d77c709e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d7db7cbd-cde9-4637-96e9-faac9cec5673" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_e9b6cc0c-cc84-412f-ab03-a3b2d77c709e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_0fad6fb9-435b-400e-a02e-b378c47bf98b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d7db7cbd-cde9-4637-96e9-faac9cec5673" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_0fad6fb9-435b-400e-a02e-b378c47bf98b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_066cf541-e002-45d7-a465-e0941aba55fa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d7db7cbd-cde9-4637-96e9-faac9cec5673" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_066cf541-e002-45d7-a465-e0941aba55fa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_2385d595-0e0f-4706-95ef-052fac7ef4c3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d7db7cbd-cde9-4637-96e9-faac9cec5673" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_2385d595-0e0f-4706-95ef-052fac7ef4c3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_f4044399-3077-41f5-9d1d-3960dc538dbd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d7db7cbd-cde9-4637-96e9-faac9cec5673" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_f4044399-3077-41f5-9d1d-3960dc538dbd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_0a833b05-8eab-417b-9783-673be583f73f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_9f500345-8ef9-4ad8-a0ad-ff8e8bc90c50" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_9f500345-8ef9-4ad8-a0ad-ff8e8bc90c50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_22c057e2-8c33-41ab-b766-59957ce25c99" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_22c057e2-8c33-41ab-b766-59957ce25c99" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_14d8dc5a-15e0-4c8c-8465-8161f9801271" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_14d8dc5a-15e0-4c8c-8465-8161f9801271" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_65626186-b7c4-4d4d-96a9-c5a4dcb0466e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_14d8dc5a-15e0-4c8c-8465-8161f9801271" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_65626186-b7c4-4d4d-96a9-c5a4dcb0466e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_054797b6-917e-47b9-9c8e-ffb9c950d159" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_14d8dc5a-15e0-4c8c-8465-8161f9801271" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_054797b6-917e-47b9-9c8e-ffb9c950d159" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_4c6c1467-f5ce-4497-9e30-b2541ad6a0cc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_14d8dc5a-15e0-4c8c-8465-8161f9801271" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_4c6c1467-f5ce-4497-9e30-b2541ad6a0cc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_8192fb6e-c089-472e-a819-b332c1f87944" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_14d8dc5a-15e0-4c8c-8465-8161f9801271" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_8192fb6e-c089-472e-a819-b332c1f87944" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_41207fc8-2513-41bc-a483-8cbb2463093d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_14d8dc5a-15e0-4c8c-8465-8161f9801271" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_41207fc8-2513-41bc-a483-8cbb2463093d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_c5e5362b-612a-4dd0-8e74-37d46cef99c7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_68bad568-7320-4a6d-83d8-1b686cf74f82" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_68bad568-7320-4a6d-83d8-1b686cf74f82" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_67cf3764-d05d-48c2-ab50-4abb35202d02" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_67cf3764-d05d-48c2-ab50-4abb35202d02" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_5a2d3623-00f9-445f-9d4b-1948e8c0184a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_5a2d3623-00f9-445f-9d4b-1948e8c0184a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_b8f3a9bc-1ed0-4d9d-82a7-117368c1bdf1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_5a2d3623-00f9-445f-9d4b-1948e8c0184a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_b8f3a9bc-1ed0-4d9d-82a7-117368c1bdf1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_b5393090-46ca-4d71-bffe-22956c365426" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_5a2d3623-00f9-445f-9d4b-1948e8c0184a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_b5393090-46ca-4d71-bffe-22956c365426" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_9f6d2ca9-c80a-4a47-8758-90ec4e9a16d4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_5a2d3623-00f9-445f-9d4b-1948e8c0184a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_9f6d2ca9-c80a-4a47-8758-90ec4e9a16d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_8adbb4bb-886c-4244-9a72-981830c974b8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_8adbb4bb-886c-4244-9a72-981830c974b8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_4698e400-14b4-4f7c-a33b-6e8f4e6eba6e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_8adbb4bb-886c-4244-9a72-981830c974b8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_4698e400-14b4-4f7c-a33b-6e8f4e6eba6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_9afdb1db-601a-4214-af92-d40a610ae72b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_bd2175d5-9f72-4055-a178-99a33de46912" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_bd2175d5-9f72-4055-a178-99a33de46912" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_cb15aee8-5aa6-40f2-b725-3e8771af99c2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_affc8feb-cf51-40a2-9236-f61576585e42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_cb15aee8-5aa6-40f2-b725-3e8771af99c2" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/StockbasedCompensationRestrictedStockUnitsDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#StockbasedCompensationRestrictedStockUnitsDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/StockbasedCompensationRestrictedStockUnitsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a48e9820-b77a-4c97-b904-7d32f49974f3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_affb90ee-ab3c-4db1-be79-af68524cb663" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a48e9820-b77a-4c97-b904-7d32f49974f3" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_affb90ee-ab3c-4db1-be79-af68524cb663" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_0daab52c-95c2-4587-8626-2895aec626e9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_affb90ee-ab3c-4db1-be79-af68524cb663" xlink:to="loc_us-gaap_AwardTypeAxis_0daab52c-95c2-4587-8626-2895aec626e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0daab52c-95c2-4587-8626-2895aec626e9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_0daab52c-95c2-4587-8626-2895aec626e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0daab52c-95c2-4587-8626-2895aec626e9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_63f5274a-3658-4fd6-82a7-3581635dfde4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_0daab52c-95c2-4587-8626-2895aec626e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_63f5274a-3658-4fd6-82a7-3581635dfde4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_6df2faac-e9b2-49da-b3bd-8914bf98323e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_63f5274a-3658-4fd6-82a7-3581635dfde4" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_6df2faac-e9b2-49da-b3bd-8914bf98323e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_8ed09a19-b34b-46d1-ba12-6ef8b0406bcf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a48e9820-b77a-4c97-b904-7d32f49974f3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_8ed09a19-b34b-46d1-ba12-6ef8b0406bcf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_f0830111-e0bb-4091-9ef5-a10df3e6fc85" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_8ed09a19-b34b-46d1-ba12-6ef8b0406bcf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_f0830111-e0bb-4091-9ef5-a10df3e6fc85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_687267f0-6b3f-4a10-b273-fe15bb96a6fd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_8ed09a19-b34b-46d1-ba12-6ef8b0406bcf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_687267f0-6b3f-4a10-b273-fe15bb96a6fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_7a4d956a-86a3-4c3a-bf88-264b113b7d15" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_8ed09a19-b34b-46d1-ba12-6ef8b0406bcf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_7a4d956a-86a3-4c3a-bf88-264b113b7d15" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_326ef27f-5a8d-47d7-85eb-960101314e51" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_8ed09a19-b34b-46d1-ba12-6ef8b0406bcf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_326ef27f-5a8d-47d7-85eb-960101314e51" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_65e23147-a697-4818-84a9-0494622d1d36" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d6fd71e8-0dab-408b-ba73-291befdba77d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_a48e9820-b77a-4c97-b904-7d32f49974f3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d6fd71e8-0dab-408b-ba73-291befdba77d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_585edee6-64cb-4504-92d9-cd3ebc242257" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d6fd71e8-0dab-408b-ba73-291befdba77d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_585edee6-64cb-4504-92d9-cd3ebc242257" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_fcc0cbe4-5a5d-4d57-be49-7e43bc0117eb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d6fd71e8-0dab-408b-ba73-291befdba77d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_fcc0cbe4-5a5d-4d57-be49-7e43bc0117eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_20e00e1d-85a3-4c0b-a770-60d21626915a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d6fd71e8-0dab-408b-ba73-291befdba77d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_20e00e1d-85a3-4c0b-a770-60d21626915a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_7b56c3a8-46f1-47e8-af3f-9c1391b40d1e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d6fd71e8-0dab-408b-ba73-291befdba77d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_7b56c3a8-46f1-47e8-af3f-9c1391b40d1e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_978b966e-2906-43f7-b8cd-ca7c393656fd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/StockbasedCompensationStockBasedCompensationDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#StockbasedCompensationStockBasedCompensationDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/StockbasedCompensationStockBasedCompensationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_a8521b62-809c-4070-b6f4-fed0b019066a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_1a772b54-9e2e-4695-b4bd-54cd5d4698ca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_a8521b62-809c-4070-b6f4-fed0b019066a" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_1a772b54-9e2e-4695-b4bd-54cd5d4698ca" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_2ea3560a-2da5-4f3b-908f-eba73a23cae4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_1a772b54-9e2e-4695-b4bd-54cd5d4698ca" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_2ea3560a-2da5-4f3b-908f-eba73a23cae4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_2ea3560a-2da5-4f3b-908f-eba73a23cae4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_2ea3560a-2da5-4f3b-908f-eba73a23cae4" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_2ea3560a-2da5-4f3b-908f-eba73a23cae4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_fb72879d-786e-41ef-9d49-bf39587cd5a1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_2ea3560a-2da5-4f3b-908f-eba73a23cae4" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_fb72879d-786e-41ef-9d49-bf39587cd5a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_42e5f9bc-8881-4e52-8ab4-aa6156a6c0df" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_fb72879d-786e-41ef-9d49-bf39587cd5a1" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_42e5f9bc-8881-4e52-8ab4-aa6156a6c0df" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_574ed042-fd98-42ee-8cf7-6a6651c6aa0a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_fb72879d-786e-41ef-9d49-bf39587cd5a1" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_574ed042-fd98-42ee-8cf7-6a6651c6aa0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAbstract_d117991a-3d9c-425c-8555-1dd8e1d9c19a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_a8521b62-809c-4070-b6f4-fed0b019066a" xlink:to="loc_us-gaap_ShareBasedCompensationAbstract_d117991a-3d9c-425c-8555-1dd8e1d9c19a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_31506cba-867b-41b7-8fa3-4ade5872f5b3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAbstract_d117991a-3d9c-425c-8555-1dd8e1d9c19a" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_31506cba-867b-41b7-8fa3-4ade5872f5b3" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/NetLossPerShareAntidilutiveCommonEquivalentSharesDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#NetLossPerShareAntidilutiveCommonEquivalentSharesDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/NetLossPerShareAntidilutiveCommonEquivalentSharesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_5bab705e-0821-43dc-b55c-e5324fa8e749" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_d75b2282-7197-42d5-994a-f506db4aa552" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_5bab705e-0821-43dc-b55c-e5324fa8e749" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_d75b2282-7197-42d5-994a-f506db4aa552" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_235d5478-d680-4301-8915-20632e4c14ba" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_d75b2282-7197-42d5-994a-f506db4aa552" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_235d5478-d680-4301-8915-20632e4c14ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_235d5478-d680-4301-8915-20632e4c14ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_235d5478-d680-4301-8915-20632e4c14ba" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_235d5478-d680-4301-8915-20632e4c14ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_9bd59792-932d-4751-849a-8429b221f71b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_235d5478-d680-4301-8915-20632e4c14ba" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_9bd59792-932d-4751-849a-8429b221f71b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_3475b4a9-03d9-4d23-8098-3307857d7510" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_9bd59792-932d-4751-849a-8429b221f71b" xlink:to="loc_us-gaap_EmployeeStockOptionMember_3475b4a9-03d9-4d23-8098-3307857d7510" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_50aad574-f60a-4b8e-9463-f33ff924ec0a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_9bd59792-932d-4751-849a-8429b221f71b" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_50aad574-f60a-4b8e-9463-f33ff924ec0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_89a803c7-cee9-4c25-b1dc-1e165dcdcf5f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_9bd59792-932d-4751-849a-8429b221f71b" xlink:to="loc_us-gaap_WarrantMember_89a803c7-cee9-4c25-b1dc-1e165dcdcf5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareReconciliationAbstract_a542a259-5c46-4d3e-9e93-fcec2245d9d8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EarningsPerShareReconciliationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_5bab705e-0821-43dc-b55c-e5324fa8e749" xlink:to="loc_us-gaap_EarningsPerShareReconciliationAbstract_a542a259-5c46-4d3e-9e93-fcec2245d9d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_80da35be-4ba7-4751-afb9-a1ec1431584f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareReconciliationAbstract_a542a259-5c46-4d3e-9e93-fcec2245d9d8" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_80da35be-4ba7-4751-afb9-a1ec1431584f" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/WarrantLiabilitiesDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#WarrantLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/WarrantLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightLineItems_221dd8e9-db23-468b-b45f-cff06f1d7925" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightTable_785e7aa0-3cd1-4ad6-a3e0-a7146d74dc59" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_221dd8e9-db23-468b-b45f-cff06f1d7925" xlink:to="loc_us-gaap_ClassOfWarrantOrRightTable_785e7aa0-3cd1-4ad6-a3e0-a7146d74dc59" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_548aa112-64b8-48e6-b35e-0eae34a99836" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfWarrantOrRightTable_785e7aa0-3cd1-4ad6-a3e0-a7146d74dc59" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_548aa112-64b8-48e6-b35e-0eae34a99836" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_548aa112-64b8-48e6-b35e-0eae34a99836_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_548aa112-64b8-48e6-b35e-0eae34a99836" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_548aa112-64b8-48e6-b35e-0eae34a99836_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_c0771c5c-cb57-454a-966c-8c27aec0be12" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_548aa112-64b8-48e6-b35e-0eae34a99836" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_c0771c5c-cb57-454a-966c-8c27aec0be12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_RedeemableWarrantsMember_5beed318-c310-4830-8408-d5a5aa67264c" xlink:href="qsi-20240930.xsd#qsi_RedeemableWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_c0771c5c-cb57-454a-966c-8c27aec0be12" xlink:to="loc_qsi_RedeemableWarrantsMember_5beed318-c310-4830-8408-d5a5aa67264c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_PrivatePlacementWarrantsMember_271feccb-e73f-4560-b8bf-ea03f7cef078" xlink:href="qsi-20240930.xsd#qsi_PrivatePlacementWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_c0771c5c-cb57-454a-966c-8c27aec0be12" xlink:to="loc_qsi_PrivatePlacementWarrantsMember_271feccb-e73f-4560-b8bf-ea03f7cef078" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WarrantsAndRightsNoteDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_221dd8e9-db23-468b-b45f-cff06f1d7925" xlink:to="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_5abb06b1-0fd6-475c-97f7-1aee3b398871" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_5abb06b1-0fd6-475c-97f7-1aee3b398871" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight_22e1f458-5245-4294-aeed-56eb948305c9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight_22e1f458-5245-4294-aeed-56eb948305c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_a89ef691-f090-411f-94bc-52d49adfcd61" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_a89ef691-f090-411f-94bc-52d49adfcd61" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_ClassOfWarrantOrRightRedemptionPrice_ba099f5b-8c1f-41cf-8909-07f04368821f" xlink:href="qsi-20240930.xsd#qsi_ClassOfWarrantOrRightRedemptionPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_qsi_ClassOfWarrantOrRightRedemptionPrice_ba099f5b-8c1f-41cf-8909-07f04368821f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_NoticePeriodToRedeemWarrants_827013ab-7d95-4e6d-b20c-d33b3d861d2a" xlink:href="qsi-20240930.xsd#qsi_NoticePeriodToRedeemWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_qsi_NoticePeriodToRedeemWarrants_827013ab-7d95-4e6d-b20c-d33b3d861d2a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_ThresholdConsecutiveTradingDays_27f024c0-bbfe-4fe2-a6db-969b72646fc3" xlink:href="qsi-20240930.xsd#qsi_ThresholdConsecutiveTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_qsi_ThresholdConsecutiveTradingDays_27f024c0-bbfe-4fe2-a6db-969b72646fc3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharePrice_682ddf85-6416-4a4c-9445-6d4d63ecc456" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_us-gaap_SharePrice_682ddf85-6416-4a4c-9445-6d4d63ecc456" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_ThresholdTradingDays_fafea00f-7eb3-4d99-996d-f94536c9f9b1" xlink:href="qsi-20240930.xsd#qsi_ThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_qsi_ThresholdTradingDays_fafea00f-7eb3-4d99-996d-f94536c9f9b1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_PeriodPriorToNoticeOfRedemption_b744aeed-f277-41f8-bc36-7fb3f35ef5a5" xlink:href="qsi-20240930.xsd#qsi_PeriodPriorToNoticeOfRedemption"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_qsi_PeriodPriorToNoticeOfRedemption_b744aeed-f277-41f8-bc36-7fb3f35ef5a5" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_TradingDayPeriodToCalculateFairMarketValueOverExercisePriceOfWarrants_28c0bad8-fa84-4ea1-bbe0-d77c46e96c6c" xlink:href="qsi-20240930.xsd#qsi_TradingDayPeriodToCalculateFairMarketValueOverExercisePriceOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_qsi_TradingDayPeriodToCalculateFairMarketValueOverExercisePriceOfWarrants_28c0bad8-fa84-4ea1-bbe0-d77c46e96c6c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_ClassOfWarrantOrRightLimitationPeriodToTransferAssignOrSellWarrants_016cd2a9-2468-468d-b77f-0f7b2f325a39" xlink:href="qsi-20240930.xsd#qsi_ClassOfWarrantOrRightLimitationPeriodToTransferAssignOrSellWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_qsi_ClassOfWarrantOrRightLimitationPeriodToTransferAssignOrSellWarrants_016cd2a9-2468-468d-b77f-0f7b2f325a39" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilitiesNoncurrent_9ca28adf-1a59-4b37-849b-4eafa33d79e7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_us-gaap_DerivativeLiabilitiesNoncurrent_9ca28adf-1a59-4b37-849b-4eafa33d79e7" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet_e3b49ddf-a56a-47a9-8621-ec746c06bd64" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WarrantsAndRightsNoteDisclosureAbstract_535c5ace-facc-4840-829d-3550d37563be" xlink:to="loc_us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet_e3b49ddf-a56a-47a9-8621-ec746c06bd64" xlink:type="arc" order="13"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/RestructuringDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#RestructuringDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/RestructuringDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_57d98e99-34b6-4672-9d8c-c2dbe602160c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_53c56a9c-175c-4a1d-b517-2ed1da2a6409" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_57d98e99-34b6-4672-9d8c-c2dbe602160c" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_53c56a9c-175c-4a1d-b517-2ed1da2a6409" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_6152061b-c18c-467a-9218-13366329bc1b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_53c56a9c-175c-4a1d-b517-2ed1da2a6409" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_6152061b-c18c-467a-9218-13366329bc1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_6152061b-c18c-467a-9218-13366329bc1b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_6152061b-c18c-467a-9218-13366329bc1b" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_6152061b-c18c-467a-9218-13366329bc1b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_fd67f921-ce18-4e1c-b678-bc5eaadb9c6c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_6152061b-c18c-467a-9218-13366329bc1b" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_fd67f921-ce18-4e1c-b678-bc5eaadb9c6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_711a3c06-f61f-4e10-a499-dc034c395aed" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_fd67f921-ce18-4e1c-b678-bc5eaadb9c6c" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_711a3c06-f61f-4e10-a499-dc034c395aed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_70140b81-dd6b-4f11-9231-48210f0a4551" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_fd67f921-ce18-4e1c-b678-bc5eaadb9c6c" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_70140b81-dd6b-4f11-9231-48210f0a4551" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveRollForward_0fdf7ca5-f01d-4ef6-9a41-6816d9f314d8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserveRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_57d98e99-34b6-4672-9d8c-c2dbe602160c" xlink:to="loc_us-gaap_RestructuringReserveRollForward_0fdf7ca5-f01d-4ef6-9a41-6816d9f314d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_4147ac85-e2fa-414d-b421-8087df38277b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_0fdf7ca5-f01d-4ef6-9a41-6816d9f314d8" xlink:to="loc_us-gaap_RestructuringReserve_4147ac85-e2fa-414d-b421-8087df38277b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_3c5ea531-5055-49e2-b492-f915d5700fe4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_0fdf7ca5-f01d-4ef6-9a41-6816d9f314d8" xlink:to="loc_us-gaap_RestructuringCharges_3c5ea531-5055-49e2-b492-f915d5700fe4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_RestructuringReservePaymentsForRestructuringAndOtherAdjustments_251a9a3e-b56a-49b0-975a-143ea076cfa2" xlink:href="qsi-20240930.xsd#qsi_RestructuringReservePaymentsForRestructuringAndOtherAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_0fdf7ca5-f01d-4ef6-9a41-6816d9f314d8" xlink:to="loc_qsi_RestructuringReservePaymentsForRestructuringAndOtherAdjustments_251a9a3e-b56a-49b0-975a-143ea076cfa2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_7c9d6f53-2075-4fbd-97ba-1ff0fde0024b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserve"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveCurrent_81aa6e88-72b5-4e9a-8470-c86f5884243c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserveCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_57d98e99-34b6-4672-9d8c-c2dbe602160c" xlink:to="loc_us-gaap_RestructuringReserveCurrent_81aa6e88-72b5-4e9a-8470-c86f5884243c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveNoncurrent_8a3219b7-2e99-4fe6-82fa-513f05fee5f3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserveNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_57d98e99-34b6-4672-9d8c-c2dbe602160c" xlink:to="loc_us-gaap_RestructuringReserveNoncurrent_8a3219b7-2e99-4fe6-82fa-513f05fee5f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_dbb2b442-6bd0-4711-bfee-aa324297ee61" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserve"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/RelatedPartyTransactionsNarrativeDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#RelatedPartyTransactionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/RelatedPartyTransactionsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_c2e2c66e-f39f-4434-8732-503949be5359" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_e1e06604-6122-4239-933f-dcac2b349299" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_c2e2c66e-f39f-4434-8732-503949be5359" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_e1e06604-6122-4239-933f-dcac2b349299" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_9a0841ce-e266-4d1b-9c8a-246089b5d512" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_e1e06604-6122-4239-933f-dcac2b349299" xlink:to="loc_srt_CounterpartyNameAxis_9a0841ce-e266-4d1b-9c8a-246089b5d512" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_9a0841ce-e266-4d1b-9c8a-246089b5d512_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_9a0841ce-e266-4d1b-9c8a-246089b5d512" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_9a0841ce-e266-4d1b-9c8a-246089b5d512_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_72206634-9a14-4d9b-82db-3d2fd0a02889" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_9a0841ce-e266-4d1b-9c8a-246089b5d512" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_72206634-9a14-4d9b-82db-3d2fd0a02889" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_Catalyzer4CorporationMember_3a1c858d-ca55-406a-ab39-c1b72a08053f" xlink:href="qsi-20240930.xsd#qsi_Catalyzer4CorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_72206634-9a14-4d9b-82db-3d2fd0a02889" xlink:to="loc_qsi_Catalyzer4CorporationMember_3a1c858d-ca55-406a-ab39-c1b72a08053f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_ProteinEvolutionIncMember_53ce84e5-65ad-4f3e-acf9-32f64205caed" xlink:href="qsi-20240930.xsd#qsi_ProteinEvolutionIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_72206634-9a14-4d9b-82db-3d2fd0a02889" xlink:to="loc_qsi_ProteinEvolutionIncMember_53ce84e5-65ad-4f3e-acf9-32f64205caed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_66a92058-084e-4da5-87c8-54245031b435" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_e1e06604-6122-4239-933f-dcac2b349299" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_66a92058-084e-4da5-87c8-54245031b435" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_66a92058-084e-4da5-87c8-54245031b435_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_66a92058-084e-4da5-87c8-54245031b435" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_66a92058-084e-4da5-87c8-54245031b435_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_a2ecb7a3-16ac-49c3-a594-f64ea3aa5aa7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_66a92058-084e-4da5-87c8-54245031b435" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_a2ecb7a3-16ac-49c3-a594-f64ea3aa5aa7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_MonthlyServicesUnderAmendedAndRestatedTechnologyServicesAgreementMember_6c4d7127-e53b-45d9-89ef-e9c9f0766e96" xlink:href="qsi-20240930.xsd#qsi_MonthlyServicesUnderAmendedAndRestatedTechnologyServicesAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_a2ecb7a3-16ac-49c3-a594-f64ea3aa5aa7" xlink:to="loc_qsi_MonthlyServicesUnderAmendedAndRestatedTechnologyServicesAgreementMember_6c4d7127-e53b-45d9-89ef-e9c9f0766e96" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_MonthToMonthSubleaseArrangementsForOfficeAndLaboratorySpacesMember_4c69c9ff-5702-4ab6-9445-9a658571e64b" xlink:href="qsi-20240930.xsd#qsi_MonthToMonthSubleaseArrangementsForOfficeAndLaboratorySpacesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_a2ecb7a3-16ac-49c3-a594-f64ea3aa5aa7" xlink:to="loc_qsi_MonthToMonthSubleaseArrangementsForOfficeAndLaboratorySpacesMember_4c69c9ff-5702-4ab6-9445-9a658571e64b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_ProteinEngineeringCollaborationMember_7f86a8c7-47a4-4c38-8551-d127fa5063c1" xlink:href="qsi-20240930.xsd#qsi_ProteinEngineeringCollaborationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_a2ecb7a3-16ac-49c3-a594-f64ea3aa5aa7" xlink:to="loc_qsi_ProteinEngineeringCollaborationMember_7f86a8c7-47a4-4c38-8551-d127fa5063c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_230a75ca-28df-444f-ac0c-41448dc16fcf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_e1e06604-6122-4239-933f-dcac2b349299" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_230a75ca-28df-444f-ac0c-41448dc16fcf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_230a75ca-28df-444f-ac0c-41448dc16fcf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_230a75ca-28df-444f-ac0c-41448dc16fcf" xlink:to="loc_us-gaap_RelatedPartyDomain_230a75ca-28df-444f-ac0c-41448dc16fcf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_3f2bed74-f0b7-4059-b09c-7f045bf59278" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_230a75ca-28df-444f-ac0c-41448dc16fcf" xlink:to="loc_us-gaap_RelatedPartyDomain_3f2bed74-f0b7-4059-b09c-7f045bf59278" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_865f1358-801c-474d-8a18-5970c2cbfe58" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_3f2bed74-f0b7-4059-b09c-7f045bf59278" xlink:to="loc_us-gaap_RelatedPartyMember_865f1358-801c-474d-8a18-5970c2cbfe58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_BoardOfDirectorsChairmanMember_30efe953-d480-4d17-a51e-2b369c474344" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_BoardOfDirectorsChairmanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_3f2bed74-f0b7-4059-b09c-7f045bf59278" xlink:to="loc_srt_BoardOfDirectorsChairmanMember_30efe953-d480-4d17-a51e-2b369c474344" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_ed79f7f8-a710-4252-bd5c-4f6e08e412ea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_e1e06604-6122-4239-933f-dcac2b349299" xlink:to="loc_us-gaap_PlanNameAxis_ed79f7f8-a710-4252-bd5c-4f6e08e412ea" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_ed79f7f8-a710-4252-bd5c-4f6e08e412ea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_ed79f7f8-a710-4252-bd5c-4f6e08e412ea" xlink:to="loc_us-gaap_PlanNameDomain_ed79f7f8-a710-4252-bd5c-4f6e08e412ea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_c5a3424f-4bf6-495c-ac0d-9097e79f16db" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_ed79f7f8-a710-4252-bd5c-4f6e08e412ea" xlink:to="loc_us-gaap_PlanNameDomain_c5a3424f-4bf6-495c-ac0d-9097e79f16db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_QuantumSiIncorporated2021EquityIncentivePlanMember_6f7d98d9-8f21-4324-b88f-432bc1144d61" xlink:href="qsi-20240930.xsd#qsi_QuantumSiIncorporated2021EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_c5a3424f-4bf6-495c-ac0d-9097e79f16db" xlink:to="loc_qsi_QuantumSiIncorporated2021EquityIncentivePlanMember_6f7d98d9-8f21-4324-b88f-432bc1144d61" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingCostsAndExpenses_ec619622-a3e7-458f-a153-5c3a4ecf528c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingCostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_c2e2c66e-f39f-4434-8732-503949be5359" xlink:to="loc_us-gaap_OperatingCostsAndExpenses_ec619622-a3e7-458f-a153-5c3a4ecf528c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherReceivablesNetCurrent_80b200dc-1115-4e8c-81ac-4db0db80edca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherReceivablesNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_c2e2c66e-f39f-4434-8732-503949be5359" xlink:to="loc_us-gaap_OtherReceivablesNetCurrent_80b200dc-1115-4e8c-81ac-4db0db80edca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesThatCanBePurchasedWithOptionGranted_8394c955-e91e-4914-93e4-c83875fbfcad" xlink:href="qsi-20240930.xsd#qsi_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesThatCanBePurchasedWithOptionGranted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_c2e2c66e-f39f-4434-8732-503949be5359" xlink:to="loc_qsi_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesThatCanBePurchasedWithOptionGranted_8394c955-e91e-4914-93e4-c83875fbfcad" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://quantum-si.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="qsi-20240930.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://quantum-si.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_b0c5bfbb-cb2d-4db3-81c7-074604a576c2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_12eeadc0-e649-4913-b08d-35258ab03fa6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_b0c5bfbb-cb2d-4db3-81c7-074604a576c2" xlink:to="loc_us-gaap_OtherCommitmentsTable_12eeadc0-e649-4913-b08d-35258ab03fa6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsAxis_76e2dce6-a752-480a-aa13-6c1545373048" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherCommitmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_12eeadc0-e649-4913-b08d-35258ab03fa6" xlink:to="loc_us-gaap_OtherCommitmentsAxis_76e2dce6-a752-480a-aa13-6c1545373048" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsDomain_76e2dce6-a752-480a-aa13-6c1545373048_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherCommitmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OtherCommitmentsAxis_76e2dce6-a752-480a-aa13-6c1545373048" xlink:to="loc_us-gaap_OtherCommitmentsDomain_76e2dce6-a752-480a-aa13-6c1545373048_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsDomain_2258ff24-9165-4f05-8bc8-939aaed560b1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherCommitmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OtherCommitmentsAxis_76e2dce6-a752-480a-aa13-6c1545373048" xlink:to="loc_us-gaap_OtherCommitmentsDomain_2258ff24-9165-4f05-8bc8-939aaed560b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_LicensesRelatedToCertainIntellectualPropertyMember_11940b79-9177-4d4b-83d4-e1a5e1b66858" xlink:href="qsi-20240930.xsd#qsi_LicensesRelatedToCertainIntellectualPropertyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_2258ff24-9165-4f05-8bc8-939aaed560b1" xlink:to="loc_qsi_LicensesRelatedToCertainIntellectualPropertyMember_11940b79-9177-4d4b-83d4-e1a5e1b66858" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsAbstract_419b9ac0-9ded-4150-96e2-17224e9fa7e0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherCommitmentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_b0c5bfbb-cb2d-4db3-81c7-074604a576c2" xlink:to="loc_us-gaap_OtherCommitmentsAbstract_419b9ac0-9ded-4150-96e2-17224e9fa7e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_OtherCommitmentAnnualMinimumFixedPayments_0f17fb73-fcaa-489d-a4be-e704fbbfa113" xlink:href="qsi-20240930.xsd#qsi_OtherCommitmentAnnualMinimumFixedPayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsAbstract_419b9ac0-9ded-4150-96e2-17224e9fa7e0" xlink:to="loc_qsi_OtherCommitmentAnnualMinimumFixedPayments_0f17fb73-fcaa-489d-a4be-e704fbbfa113" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentDueInNextTwelveMonths_bde16dbf-c314-4305-b11a-f30b39c17952" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherCommitmentDueInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsAbstract_419b9ac0-9ded-4150-96e2-17224e9fa7e0" xlink:to="loc_us-gaap_OtherCommitmentDueInNextTwelveMonths_bde16dbf-c314-4305-b11a-f30b39c17952" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CompensationAndRetirementDisclosureAbstract_ca32e7b0-58d8-43fc-a2db-c03e550ff6be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CompensationAndRetirementDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_b0c5bfbb-cb2d-4db3-81c7-074604a576c2" xlink:to="loc_us-gaap_CompensationAndRetirementDisclosureAbstract_ca32e7b0-58d8-43fc-a2db-c03e550ff6be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount_2cb1e5ff-fcab-4e6f-9120-11268a0e25c5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CompensationAndRetirementDisclosureAbstract_ca32e7b0-58d8-43fc-a2db-c03e550ff6be" xlink:to="loc_us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount_2cb1e5ff-fcab-4e6f-9120-11268a0e25c5" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2024/ecd-2024.xsd#AwardTypeOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2024/ecd-2024.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2024/ecd-2024.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_qsi_JonathanM.RothbergMember_fd19284f-5741-4378-bc64-c9d199bb5816" xlink:href="qsi-20240930.xsd#qsi_JonathanM.RothbergMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_qsi_JonathanM.RothbergMember_fd19284f-5741-4378-bc64-c9d199bb5816" xlink:type="arc" order="2"/>
  </link:definitionLink>
</link:linkbase>
