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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e22b2-18ca-702c-b0d4-ec367c7f2db9,g:693ef38f-f8b2-4baf-9cb4-a0160475422f-->
<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://www.qualys.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="qlys-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e22b2-1a2c-7eb9-811d-0119b951d242" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e22b2-1a2c-7955-b2ae-e286a62f88fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e22b2-1a2c-7eb9-811d-0119b951d242" xlink:to="loc_us-gaap_StatementTable_019e22b2-1a2c-7955-b2ae-e286a62f88fd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7991-b05f-f46cd8b2ad61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e22b2-1a2c-7955-b2ae-e286a62f88fd" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7991-b05f-f46cd8b2ad61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7991-b05f-f46cd8b2ad61_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7991-b05f-f46cd8b2ad61" xlink:to="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7991-b05f-f46cd8b2ad61_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7708-9caf-f3ee40632bd7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7991-b05f-f46cd8b2ad61" xlink:to="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7708-9caf-f3ee40632bd7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e22b2-1a2c-7084-95a2-0656cc9f86d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7708-9caf-f3ee40632bd7" xlink:to="loc_us-gaap_CommonStockMember_019e22b2-1a2c-7084-95a2-0656cc9f86d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e22b2-1a2c-7d91-b707-a380eb4a7f3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7708-9caf-f3ee40632bd7" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e22b2-1a2c-7d91-b707-a380eb4a7f3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-76ea-84de-182750878f02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7708-9caf-f3ee40632bd7" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-76ea-84de-182750878f02" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e22b2-1a2c-73b3-b2e2-a5c9b9a45a86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7708-9caf-f3ee40632bd7" xlink:to="loc_us-gaap_RetainedEarningsMember_019e22b2-1a2c-73b3-b2e2-a5c9b9a45a86" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e22b2-1a2c-7eb9-811d-0119b951d242" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019e22b2-1a2c-7128-8a91-9b4297f4f9a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_SharesOutstanding_019e22b2-1a2c-7128-8a91-9b4297f4f9a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e22b2-1a2c-71d4-90cb-e17bb642cd03" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_StockholdersEquity_019e22b2-1a2c-71d4-90cb-e17bb642cd03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e22b2-1a2c-789f-b64e-14ec29f376ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_NetIncomeLoss_019e22b2-1a2c-789f-b64e-14ec29f376ac" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e22b2-1a2c-711d-8f9a-998d39a834d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e22b2-1a2c-711d-8f9a-998d39a834d5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e22b2-1a2c-7c24-aebf-74e3b3b3e3f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e22b2-1a2c-7c24-aebf-74e3b3b3e3f8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e22b2-1a2c-7d60-9058-7e9f6f172951" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e22b2-1a2c-7d60-9058-7e9f6f172951" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e22b2-1a2c-761f-bfe1-53a862a50fd4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e22b2-1a2c-761f-bfe1-53a862a50fd4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019e22b2-1a2c-714e-bc72-e78b106b743c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019e22b2-1a2c-714e-bc72-e78b106b743c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings_019e22b2-1a2c-7dd4-b6a5-7e69d841f959" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings_019e22b2-1a2c-7dd4-b6a5-7e69d841f959" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019e22b2-1a2c-74b4-b15c-0141a3c33e3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019e22b2-1a2c-74b4-b15c-0141a3c33e3d" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019e22b2-1a2c-7f71-a435-ed6b1479346e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019e22b2-1a2c-7f71-a435-ed6b1479346e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e22b2-1a2c-7774-98db-72d1e0974efd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e22b2-1a2c-7774-98db-72d1e0974efd" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019e22b2-1a2c-7ed9-9c92-dc511b82e1b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019e22b2-1a2c-7ed9-9c92-dc511b82e1b4" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e22b2-1a2c-7be9-966b-d14ed6183568" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e22b2-1a2c-700d-bbaa-e7a1e667d5d7" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e22b2-1a2c-7be9-966b-d14ed6183568" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019e22b2-1a2c-7a1b-8fa5-d68ab5dfe1cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e22b2-1a2c-7249-a471-1abcc6f4207b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/FairValueofFinancialInstrumentsScheduleofAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#FairValueofFinancialInstrumentsScheduleofAssetsandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/FairValueofFinancialInstrumentsScheduleofAssetsandLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-70a4-af8b-fc6c9d8aae42" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-70a4-af8b-fc6c9d8aae42" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e22b2-1a2c-78bc-bff3-3660b051a299" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-70a4-af8b-fc6c9d8aae42" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e22b2-1a2c-78bc-bff3-3660b051a299" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e22b2-1a2c-78bc-bff3-3660b051a299_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e22b2-1a2c-78bc-bff3-3660b051a299" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e22b2-1a2c-78bc-bff3-3660b051a299_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e22b2-1a2c-76da-b73d-d1b68f43d73f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e22b2-1a2c-78bc-bff3-3660b051a299" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e22b2-1a2c-76da-b73d-d1b68f43d73f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019e22b2-1a2c-7ef4-b2ed-192ef259ba05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e22b2-1a2c-76da-b73d-d1b68f43d73f" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019e22b2-1a2c-7ef4-b2ed-192ef259ba05" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e22b2-1a2c-7ed7-9a4f-d572d65f69ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e22b2-1a2c-76da-b73d-d1b68f43d73f" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e22b2-1a2c-7ed7-9a4f-d572d65f69ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsAtCarryingValue_019e22b2-1a2c-73e6-ae5c-5b50870fbadb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_MoneyMarketFundsAtCarryingValue_019e22b2-1a2c-73e6-ae5c-5b50870fbadb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperAtCarryingValue_019e22b2-1a2c-78c3-86fa-df186f9b1438" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_CommercialPaperAtCarryingValue_019e22b2-1a2c-78c3-86fa-df186f9b1438" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesSecuritiesAtCarryingValue_019e22b2-1a2c-7e69-b206-031b639451f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesSecuritiesAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_USGovernmentAgenciesSecuritiesAtCarryingValue_019e22b2-1a2c-7e69-b206-031b639451f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsOwnedCorporateDebtAtFairValue_019e22b2-1a2c-7b75-8289-c5dcb38270fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsOwnedCorporateDebtAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_FinancialInstrumentsOwnedCorporateDebtAtFairValue_019e22b2-1a2c-7b75-8289-c5dcb38270fd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetBackedSecuritiesAtCarryingValue_019e22b2-1a2c-7194-b955-23ea115ba281" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetBackedSecuritiesAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_AssetBackedSecuritiesAtCarryingValue_019e22b2-1a2c-7194-b955-23ea115ba281" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsOwnedDerivativesAndOtherContractualAgreementsAtFairValue_019e22b2-1a2c-731c-81b3-693c8e9e0c05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsOwnedDerivativesAndOtherContractualAgreementsAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_FinancialInstrumentsOwnedDerivativesAndOtherContractualAgreementsAtFairValue_019e22b2-1a2c-731c-81b3-693c8e9e0c05" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsOwnedAtFairValue_019e22b2-1a2c-7aa9-9e06-7bb6ae476b8a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_FinancialInstrumentsOwnedAtFairValue_019e22b2-1a2c-7aa9-9e06-7bb6ae476b8a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_019e22b2-1a2c-7b00-b462-729844325e97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_019e22b2-1a2c-7b00-b462-729844325e97" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialLiabilitiesFairValueDisclosure_019e22b2-1a2c-7e18-a284-3461acd4d41d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialLiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-73c4-ba99-a28db117ee05" xlink:to="loc_us-gaap_FinancialLiabilitiesFairValueDisclosure_019e22b2-1a2c-7e18-a284-3461acd4d41d" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/FairValueofFinancialInstrumentsScheduleofCashandCashEquivalentsDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#FairValueofFinancialInstrumentsScheduleofCashandCashEquivalentsDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/FairValueofFinancialInstrumentsScheduleofCashandCashEquivalentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7d1e-bfd4-fd039e54db57" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e40-9426-7a5250f9c590" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7d1e-bfd4-fd039e54db57" xlink:to="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e40-9426-7a5250f9c590" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019e22b2-1a2c-7e63-9dd4-cd63be7d1479" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e40-9426-7a5250f9c590" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019e22b2-1a2c-7e63-9dd4-cd63be7d1479" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e22b2-1a2c-7e63-9dd4-cd63be7d1479_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e22b2-1a2c-7e63-9dd4-cd63be7d1479" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e22b2-1a2c-7e63-9dd4-cd63be7d1479_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e22b2-1a2c-723f-b5a9-fc4c188b7f2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e22b2-1a2c-7e63-9dd4-cd63be7d1479" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e22b2-1a2c-723f-b5a9-fc4c188b7f2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019e22b2-1a2c-7678-80ce-543ece0cad2e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e22b2-1a2c-723f-b5a9-fc4c188b7f2a" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019e22b2-1a2c-7678-80ce-543ece0cad2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember_019e22b2-1a2c-72fa-95d2-47f0a7bc438a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e22b2-1a2c-723f-b5a9-fc4c188b7f2a" xlink:to="loc_us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember_019e22b2-1a2c-72fa-95d2-47f0a7bc438a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e22b2-1a2c-79ad-aec0-8808f70a086e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e40-9426-7a5250f9c590" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e22b2-1a2c-79ad-aec0-8808f70a086e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-79ad-aec0-8808f70a086e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e22b2-1a2c-79ad-aec0-8808f70a086e" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-79ad-aec0-8808f70a086e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-7870-bfb7-748808dc707b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e22b2-1a2c-79ad-aec0-8808f70a086e" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-7870-bfb7-748808dc707b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019e22b2-1a2c-72c0-8cdd-206441f25069" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-7870-bfb7-748808dc707b" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019e22b2-1a2c-72c0-8cdd-206441f25069" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e22b2-1a2c-7148-98f7-79b16bf36f98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-7870-bfb7-748808dc707b" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e22b2-1a2c-7148-98f7-79b16bf36f98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetBackedSecuritiesMember_019e22b2-1a2c-7f33-84e2-14fd0a2f6d2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-7870-bfb7-748808dc707b" xlink:to="loc_us-gaap_AssetBackedSecuritiesMember_019e22b2-1a2c-7f33-84e2-14fd0a2f6d2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e22b2-1a2c-7512-bf42-b7635d491fee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-7870-bfb7-748808dc707b" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e22b2-1a2c-7512-bf42-b7635d491fee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019e22b2-1a2c-742e-afa6-f30573b4c037" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e40-9426-7a5250f9c590" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019e22b2-1a2c-742e-afa6-f30573b4c037" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e22b2-1a2c-742e-afa6-f30573b4c037_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e22b2-1a2c-742e-afa6-f30573b4c037" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e22b2-1a2c-742e-afa6-f30573b4c037_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e22b2-1a2c-7445-b073-81287babb85c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019e22b2-1a2c-742e-afa6-f30573b4c037" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e22b2-1a2c-7445-b073-81287babb85c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsMember_019e22b2-1a2c-785a-af65-5c9f958ce5a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e22b2-1a2c-7445-b073-81287babb85c" xlink:to="loc_us-gaap_CashEquivalentsMember_019e22b2-1a2c-785a-af65-5c9f958ce5a6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019e22b2-1a2c-7df7-ba31-632073474dad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e22b2-1a2c-7445-b073-81287babb85c" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019e22b2-1a2c-7df7-ba31-632073474dad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLongTermInvestmentsMember_019e22b2-1a2c-72eb-a94c-d7a831e0754d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLongTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019e22b2-1a2c-7445-b073-81287babb85c" xlink:to="loc_us-gaap_OtherLongTermInvestmentsMember_019e22b2-1a2c-72eb-a94c-d7a831e0754d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss_019e22b2-1a2c-7b1d-8681-3f9e0e604dd9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7d1e-bfd4-fd039e54db57" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss_019e22b2-1a2c-7b1d-8681-3f9e0e604dd9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e22b2-1a2c-7cb1-8a9a-ff55b37d9b31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7d1e-bfd4-fd039e54db57" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e22b2-1a2c-7cb1-8a9a-ff55b37d9b31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e22b2-1a2c-7e76-a660-a1aaf2fda50c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7d1e-bfd4-fd039e54db57" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e22b2-1a2c-7e76-a660-a1aaf2fda50c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019e22b2-1a2c-7726-97e3-1296543c0beb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7d1e-bfd4-fd039e54db57" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019e22b2-1a2c-7726-97e3-1296543c0beb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Cash_019e22b2-1a2c-7183-862c-9146a55d621c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Cash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7d1e-bfd4-fd039e54db57" xlink:to="loc_us-gaap_Cash_019e22b2-1a2c-7183-862c-9146a55d621c" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/FairValueofFinancialInstrumentsScheduleofSecuritiesinUnrealizedLossPositionsDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#FairValueofFinancialInstrumentsScheduleofSecuritiesinUnrealizedLossPositionsDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/FairValueofFinancialInstrumentsScheduleofSecuritiesinUnrealizedLossPositionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-75e5-887c-4cdd81fc3091" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-735d-b39a-8f09ba03da0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-75e5-887c-4cdd81fc3091" xlink:to="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-735d-b39a-8f09ba03da0a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e22b2-1a2c-7e5e-b5cd-9a50755469bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-735d-b39a-8f09ba03da0a" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e22b2-1a2c-7e5e-b5cd-9a50755469bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-7e5e-b5cd-9a50755469bf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e22b2-1a2c-7e5e-b5cd-9a50755469bf" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-7e5e-b5cd-9a50755469bf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-719a-88d9-e916351dc448" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e22b2-1a2c-7e5e-b5cd-9a50755469bf" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-719a-88d9-e916351dc448" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019e22b2-1a2c-7d9d-9b4b-84ff8f69b9db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-719a-88d9-e916351dc448" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019e22b2-1a2c-7d9d-9b4b-84ff8f69b9db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e22b2-1a2c-76f9-a329-d2ce81878157" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-719a-88d9-e916351dc448" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e22b2-1a2c-76f9-a329-d2ce81878157" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e22b2-1a2c-7392-a804-8161a36c3b32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e22b2-1a2c-719a-88d9-e916351dc448" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e22b2-1a2c-7392-a804-8161a36c3b32" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesCurrentAbstract_019e22b2-1a2c-75c4-9d85-88bc305e4666" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-75e5-887c-4cdd81fc3091" xlink:to="loc_us-gaap_DebtSecuritiesCurrentAbstract_019e22b2-1a2c-75c4-9d85-88bc305e4666" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019e22b2-1a2c-7bbd-bc5a-496beb9b9948" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesCurrentAbstract_019e22b2-1a2c-75c4-9d85-88bc305e4666" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019e22b2-1a2c-7bbd-bc5a-496beb9b9948" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019e22b2-1a2c-7966-abae-08cc471e6273" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesCurrentAbstract_019e22b2-1a2c-75c4-9d85-88bc305e4666" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019e22b2-1a2c-7966-abae-08cc471e6273" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesNoncurrentAbstract_019e22b2-1a2c-740f-9245-a7ce5f6cfb7e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesNoncurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-75e5-887c-4cdd81fc3091" xlink:to="loc_us-gaap_DebtSecuritiesNoncurrentAbstract_019e22b2-1a2c-740f-9245-a7ce5f6cfb7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019e22b2-1a2c-7408-8de3-f91f8a7b6f8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesNoncurrentAbstract_019e22b2-1a2c-740f-9245-a7ce5f6cfb7e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019e22b2-1a2c-7408-8de3-f91f8a7b6f8f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019e22b2-1a2c-7de3-a310-a4a39bdfe7da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesNoncurrentAbstract_019e22b2-1a2c-740f-9245-a7ce5f6cfb7e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019e22b2-1a2c-7de3-a310-a4a39bdfe7da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAbstract_019e22b2-1a2c-7bc3-b22f-30733ee75633" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-75e5-887c-4cdd81fc3091" xlink:to="loc_us-gaap_DebtSecuritiesAbstract_019e22b2-1a2c-7bc3-b22f-30733ee75633" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019e22b2-1a2c-7b45-933f-c9976f3161f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAbstract_019e22b2-1a2c-7bc3-b22f-30733ee75633" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019e22b2-1a2c-7b45-933f-c9976f3161f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019e22b2-1a2c-772b-a650-5645ddd23845" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAbstract_019e22b2-1a2c-7bc3-b22f-30733ee75633" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019e22b2-1a2c-772b-a650-5645ddd23845" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/FairValueofFinancialInstrumentsNarrativeDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#FairValueofFinancialInstrumentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/FairValueofFinancialInstrumentsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e42-b58f-a8b1ffb6453a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:to="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e42-b58f-a8b1ffb6453a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e22b2-1a2c-7c2f-bf5a-97899b6f4322" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e42-b58f-a8b1ffb6453a" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e22b2-1a2c-7c2f-bf5a-97899b6f4322" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e22b2-1a2c-7c2f-bf5a-97899b6f4322_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e22b2-1a2c-7c2f-bf5a-97899b6f4322" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e22b2-1a2c-7c2f-bf5a-97899b6f4322_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e22b2-1a2c-7cbf-856f-809cb62ec05b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e22b2-1a2c-7c2f-bf5a-97899b6f4322" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e22b2-1a2c-7cbf-856f-809cb62ec05b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeContractMember_019e22b2-1a2c-76f0-b005-783648b157de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignExchangeContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e22b2-1a2c-7cbf-856f-809cb62ec05b" xlink:to="loc_us-gaap_ForeignExchangeContractMember_019e22b2-1a2c-76f0-b005-783648b157de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ForeignExchangeContractsForGBPAndEuroMember_019e22b2-1a2c-782a-a63d-833b398de7e7" xlink:href="qlys-20260630.xsd#qlys_ForeignExchangeContractsForGBPAndEuroMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e22b2-1a2c-7cbf-856f-809cb62ec05b" xlink:to="loc_qlys_ForeignExchangeContractsForGBPAndEuroMember_019e22b2-1a2c-782a-a63d-833b398de7e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ForeignExchangeContractsForINRMember_019e22b2-1a2c-7544-af59-c2d8bf00781c" xlink:href="qlys-20260630.xsd#qlys_ForeignExchangeContractsForINRMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e22b2-1a2c-7cbf-856f-809cb62ec05b" xlink:to="loc_qlys_ForeignExchangeContractsForINRMember_019e22b2-1a2c-7544-af59-c2d8bf00781c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e22b2-1a2c-70b4-a4d9-ef7e1e9f39d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e42-b58f-a8b1ffb6453a" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e22b2-1a2c-70b4-a4d9-ef7e1e9f39d5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-70b4-a4d9-ef7e1e9f39d5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e22b2-1a2c-70b4-a4d9-ef7e1e9f39d5" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-70b4-a4d9-ef7e1e9f39d5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-7f54-874a-61eba9bb4781" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e22b2-1a2c-70b4-a4d9-ef7e1e9f39d5" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-7f54-874a-61eba9bb4781" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e22b2-1a2c-770f-961c-4fe7b544119e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-7f54-874a-61eba9bb4781" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e22b2-1a2c-770f-961c-4fe7b544119e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019e22b2-1a2c-7af6-a1d9-2b492878506f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-7f54-874a-61eba9bb4781" xlink:to="loc_us-gaap_NondesignatedMember_019e22b2-1a2c-7af6-a1d9-2b492878506f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019e22b2-1a2c-7224-918d-bd4bcd2fbff3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-7e42-b58f-a8b1ffb6453a" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019e22b2-1a2c-7224-918d-bd4bcd2fbff3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_019e22b2-1a2c-7224-918d-bd4bcd2fbff3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019e22b2-1a2c-7224-918d-bd4bcd2fbff3" xlink:to="loc_us-gaap_HedgingRelationshipDomain_019e22b2-1a2c-7224-918d-bd4bcd2fbff3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_019e22b2-1a2c-70e0-9f01-2d3faa688637" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019e22b2-1a2c-7224-918d-bd4bcd2fbff3" xlink:to="loc_us-gaap_HedgingRelationshipDomain_019e22b2-1a2c-70e0-9f01-2d3faa688637" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowHedgingMember_019e22b2-1a2c-7626-9e93-48a978ae76e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashFlowHedgingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_019e22b2-1a2c-70e0-9f01-2d3faa688637" xlink:to="loc_us-gaap_CashFlowHedgingMember_019e22b2-1a2c-7626-9e93-48a978ae76e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuritiesBorrowedCreditLossExpenseReversal_019e22b2-1a2c-7506-9e23-6d16f54c0733" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuritiesBorrowedCreditLossExpenseReversal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:to="loc_us-gaap_SecuritiesBorrowedCreditLossExpenseReversal_019e22b2-1a2c-7506-9e23-6d16f54c0733" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019e22b2-1a2c-72a8-a319-f869d6bf8ee6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019e22b2-1a2c-72a8-a319-f869d6bf8ee6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount_019e22b2-1a2c-7e58-bd7f-3aa1dfa98367" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount_019e22b2-1a2c-7e58-bd7f-3aa1dfa98367" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019e22b2-1a2c-7dbb-ad97-cb9fdbbc2857" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019e22b2-1a2c-7dbb-ad97-cb9fdbbc2857" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyCashFlowHedgeGainLossToBeReclassifiedDuringNext12Months_019e22b2-1a2c-74d5-9453-0cc87e919311" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyCashFlowHedgeGainLossToBeReclassifiedDuringNext12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:to="loc_us-gaap_ForeignCurrencyCashFlowHedgeGainLossToBeReclassifiedDuringNext12Months_019e22b2-1a2c-74d5-9453-0cc87e919311" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetFairValueGrossLiability_019e22b2-1a2c-7777-8185-5250d6740c7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssetFairValueGrossLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:to="loc_us-gaap_DerivativeAssetFairValueGrossLiability_019e22b2-1a2c-7777-8185-5250d6740c7a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityFairValueGrossAsset_019e22b2-1a2c-72f7-a957-9fe7245c4140" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilityFairValueGrossAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-7012-a19f-b8bbe7a00ad6" xlink:to="loc_us-gaap_DerivativeLiabilityFairValueGrossAsset_019e22b2-1a2c-72f7-a957-9fe7245c4140" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/FairValueofFinancialInstrumentsScheduleofDerivativeFinancialInstrumentsDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#FairValueofFinancialInstrumentsScheduleofDerivativeFinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/FairValueofFinancialInstrumentsScheduleofDerivativeFinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-76de-96ee-331404aae57f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-71cb-b789-e1fe7d616ee7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-76de-96ee-331404aae57f" xlink:to="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-71cb-b789-e1fe7d616ee7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e22b2-1a2c-721a-9dda-4a4574d5d56f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueDisclosureOfAssetAndLiabilityNotMeasuredAtFairValueTable_019e22b2-1a2c-71cb-b789-e1fe7d616ee7" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e22b2-1a2c-721a-9dda-4a4574d5d56f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-721a-9dda-4a4574d5d56f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e22b2-1a2c-721a-9dda-4a4574d5d56f" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-721a-9dda-4a4574d5d56f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-7e69-b6c5-2ae3e1afb12c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e22b2-1a2c-721a-9dda-4a4574d5d56f" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-7e69-b6c5-2ae3e1afb12c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e22b2-1a2c-7b0d-b278-b29b6e886f4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-7e69-b6c5-2ae3e1afb12c" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e22b2-1a2c-7b0d-b278-b29b6e886f4b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019e22b2-1a2c-7081-ae5c-540c12bf6327" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e22b2-1a2c-7e69-b6c5-2ae3e1afb12c" xlink:to="loc_us-gaap_NondesignatedMember_019e22b2-1a2c-7081-ae5c-540c12bf6327" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyContractAssetFairValueDisclosure_019e22b2-1a2c-725b-a7ba-ca0cef57ae64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyContractAssetFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-76de-96ee-331404aae57f" xlink:to="loc_us-gaap_ForeignCurrencyContractAssetFairValueDisclosure_019e22b2-1a2c-725b-a7ba-ca0cef57ae64" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_019e22b2-1a2c-7e95-91c9-aad98e4a58ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureAssetAndLiabilityNotMeasuredAtFairValueLineItems_019e22b2-1a2c-76de-96ee-331404aae57f" xlink:to="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_019e22b2-1a2c-7e95-91c9-aad98e4a58ee" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/AccumulatedOtherComprehensiveIncomeLossScheduleofComponentsandChangesDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#AccumulatedOtherComprehensiveIncomeLossScheduleofComponentsandChangesDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/AccumulatedOtherComprehensiveIncomeLossScheduleofComponentsandChangesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-7efe-a6a3-e7c5f610abd3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-71c2-9cdf-7b2ac855146e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-7efe-a6a3-e7c5f610abd3" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-71c2-9cdf-7b2ac855146e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-76c4-bffa-1bf308adf25b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-71c2-9cdf-7b2ac855146e" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-76c4-bffa-1bf308adf25b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-76c4-bffa-1bf308adf25b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-76c4-bffa-1bf308adf25b" xlink:to="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-76c4-bffa-1bf308adf25b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7f17-8cd2-2a6429723924" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-76c4-bffa-1bf308adf25b" xlink:to="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7f17-8cd2-2a6429723924" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-7a4a-87a8-8c6d26b8120c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7f17-8cd2-2a6429723924" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-7a4a-87a8-8c6d26b8120c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019e22b2-1a2c-7160-b9b7-0b8dc874b799" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-7a4a-87a8-8c6d26b8120c" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019e22b2-1a2c-7160-b9b7-0b8dc874b799" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019e22b2-1a2c-753b-bd02-51b9cd0e18e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-7a4a-87a8-8c6d26b8120c" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019e22b2-1a2c-753b-bd02-51b9cd0e18e0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e22b2-1a2c-7a36-bfa4-500c92f6471e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AOCIAttributableToParentNetOfTaxRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-7efe-a6a3-e7c5f610abd3" xlink:to="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e22b2-1a2c-7a36-bfa4-500c92f6471e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e22b2-1a2c-75c3-be67-5456071d6aaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e22b2-1a2c-7a36-bfa4-500c92f6471e" xlink:to="loc_us-gaap_StockholdersEquity_019e22b2-1a2c-75c3-be67-5456071d6aaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_019e22b2-1a2c-7e3f-90f1-d1474bde6a94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e22b2-1a2c-7a36-bfa4-500c92f6471e" xlink:to="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_019e22b2-1a2c-7e3f-90f1-d1474bde6a94" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_019e22b2-1a2c-70d0-9233-b2523699b0e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e22b2-1a2c-7a36-bfa4-500c92f6471e" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_019e22b2-1a2c-70d0-9233-b2523699b0e0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_019e22b2-1a2c-7ded-9293-6ff8b0aa5266" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e22b2-1a2c-7a36-bfa4-500c92f6471e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_019e22b2-1a2c-7ded-9293-6ff8b0aa5266" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e22b2-1a2c-7550-85d7-b5325ac66f21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_019e22b2-1a2c-7a36-bfa4-500c92f6471e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e22b2-1a2c-7550-85d7-b5325ac66f21" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e22b2-1a2c-7215-a457-431b8b3076c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/AccumulatedOtherComprehensiveIncomeLossScheduleofAmountsReclassifiedfromAOCIDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#AccumulatedOtherComprehensiveIncomeLossScheduleofAmountsReclassifiedfromAOCIDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/AccumulatedOtherComprehensiveIncomeLossScheduleofAmountsReclassifiedfromAOCIDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-769e-b2e4-2c935874df98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-7e1b-bf94-2e32e01d9d36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-769e-b2e4-2c935874df98" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-7e1b-bf94-2e32e01d9d36" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e22b2-1a2c-7b20-914d-d9c655a7e358" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-7e1b-bf94-2e32e01d9d36" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e22b2-1a2c-7b20-914d-d9c655a7e358" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-7b20-914d-d9c655a7e358_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e22b2-1a2c-7b20-914d-d9c655a7e358" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-7b20-914d-d9c655a7e358_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-70b2-be21-3a5c40474f36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e22b2-1a2c-7b20-914d-d9c655a7e358" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-70b2-be21-3a5c40474f36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-7325-ad01-d71ed96e83bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-70b2-be21-3a5c40474f36" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-7325-ad01-d71ed96e83bd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7086-a8d0-96e2beaf041b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-7e1b-bf94-2e32e01d9d36" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7086-a8d0-96e2beaf041b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7086-a8d0-96e2beaf041b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7086-a8d0-96e2beaf041b" xlink:to="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7086-a8d0-96e2beaf041b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-73c7-9e5c-ba3b8bd13799" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7086-a8d0-96e2beaf041b" xlink:to="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-73c7-9e5c-ba3b8bd13799" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019e22b2-1a2c-720e-a4c0-8d765fd33abe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-73c7-9e5c-ba3b8bd13799" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019e22b2-1a2c-720e-a4c0-8d765fd33abe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AOCIAttributableToParentAbstract_019e22b2-1a2c-729d-90c2-272fac0510ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AOCIAttributableToParentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-769e-b2e4-2c935874df98" xlink:to="loc_us-gaap_AOCIAttributableToParentAbstract_019e22b2-1a2c-729d-90c2-272fac0510ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e22b2-1a2c-7fec-84b9-b3d7f00e0ce9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentAbstract_019e22b2-1a2c-729d-90c2-272fac0510ca" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e22b2-1a2c-7fec-84b9-b3d7f00e0ce9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_019e22b2-1a2c-732f-8f7d-1318c20cb772" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentAbstract_019e22b2-1a2c-729d-90c2-272fac0510ca" xlink:to="loc_us-gaap_CostOfRevenue_019e22b2-1a2c-732f-8f7d-1318c20cb772" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_019e22b2-1a2c-7fa2-bb59-a0cbf387f9d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentAbstract_019e22b2-1a2c-729d-90c2-272fac0510ca" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_019e22b2-1a2c-7fa2-bb59-a0cbf387f9d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpense_019e22b2-1a2c-760b-9660-a0f8a6a41abb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingAndMarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentAbstract_019e22b2-1a2c-729d-90c2-272fac0510ca" xlink:to="loc_us-gaap_SellingAndMarketingExpense_019e22b2-1a2c-760b-9660-a0f8a6a41abb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_019e22b2-1a2c-7a19-80a2-ebc85da0c7a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentAbstract_019e22b2-1a2c-729d-90c2-272fac0510ca" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_019e22b2-1a2c-7a19-80a2-ebc85da0c7a0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e22b2-1a2c-7146-90f0-a1f32871dad8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentAbstract_019e22b2-1a2c-729d-90c2-272fac0510ca" xlink:to="loc_us-gaap_NetIncomeLoss_019e22b2-1a2c-7146-90f0-a1f32871dad8" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/AccumulatedOtherComprehensiveIncomeLossNarrativeDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#AccumulatedOtherComprehensiveIncomeLossNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/AccumulatedOtherComprehensiveIncomeLossNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-7513-92f7-f21d901ebc9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-75de-bd4a-c9224f6f551f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-7513-92f7-f21d901ebc9f" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-75de-bd4a-c9224f6f551f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e22b2-1a2c-7f15-9589-2782a5f48756" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-75de-bd4a-c9224f6f551f" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e22b2-1a2c-7f15-9589-2782a5f48756" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-7f15-9589-2782a5f48756_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e22b2-1a2c-7f15-9589-2782a5f48756" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-7f15-9589-2782a5f48756_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-7b55-95b3-c4830a7bf5ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019e22b2-1a2c-7f15-9589-2782a5f48756" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-7b55-95b3-c4830a7bf5ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-7435-a84e-42dfa6ad27d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019e22b2-1a2c-7b55-95b3-c4830a7bf5ca" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019e22b2-1a2c-7435-a84e-42dfa6ad27d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7244-ad33-1347f84a01af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019e22b2-1a2c-75de-bd4a-c9224f6f551f" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7244-ad33-1347f84a01af" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7244-ad33-1347f84a01af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7244-ad33-1347f84a01af" xlink:to="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7244-ad33-1347f84a01af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7653-82ee-81d9728ef970" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e22b2-1a2c-7244-ad33-1347f84a01af" xlink:to="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7653-82ee-81d9728ef970" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019e22b2-1a2c-7fa3-ab45-0688d9fa9770" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e22b2-1a2c-7653-82ee-81d9728ef970" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_019e22b2-1a2c-7fa3-ab45-0688d9fa9770" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019e22b2-1a2c-7c0a-90d4-aa252b1540c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019e22b2-1a2c-7513-92f7-f21d901ebc9f" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019e22b2-1a2c-7c0a-90d4-aa252b1540c1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/PropertyandEquipmentNetScheduleofPropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#PropertyandEquipmentNetScheduleofPropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/PropertyandEquipmentNetScheduleofPropertyandEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7447-a981-9fd927a55aba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e22b2-1a2c-7b49-95b3-9f92ca0264aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7447-a981-9fd927a55aba" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e22b2-1a2c-7b49-95b3-9f92ca0264aa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e22b2-1a2c-7a80-900f-4bb1146f82ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e22b2-1a2c-7b49-95b3-9f92ca0264aa" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e22b2-1a2c-7a80-900f-4bb1146f82ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-7a80-900f-4bb1146f82ed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e22b2-1a2c-7a80-900f-4bb1146f82ed" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-7a80-900f-4bb1146f82ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-787f-b29a-236a190a79df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e22b2-1a2c-7a80-900f-4bb1146f82ed" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-787f-b29a-236a190a79df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_019e22b2-1a2c-7951-8f50-9619f83e9556" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-787f-b29a-236a190a79df" xlink:to="loc_us-gaap_ComputerEquipmentMember_019e22b2-1a2c-7951-8f50-9619f83e9556" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_019e22b2-1a2c-7a21-8b08-6c0ea721eca6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-787f-b29a-236a190a79df" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_019e22b2-1a2c-7a21-8b08-6c0ea721eca6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_019e22b2-1a2c-7b51-bad4-ab541dbb8de9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-787f-b29a-236a190a79df" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_019e22b2-1a2c-7b51-bad4-ab541dbb8de9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ScannerAppliancesMember_019e22b2-1a2c-7a01-95b2-2db372ba7c96" xlink:href="qlys-20260630.xsd#qlys_ScannerAppliancesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-787f-b29a-236a190a79df" xlink:to="loc_qlys_ScannerAppliancesMember_019e22b2-1a2c-7a01-95b2-2db372ba7c96" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_019e22b2-1a2c-7b11-bcc7-d76ace68fb0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-787f-b29a-236a190a79df" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_019e22b2-1a2c-7b11-bcc7-d76ace68fb0b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019e22b2-1a2c-769f-b41d-eb74a6fde4c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7447-a981-9fd927a55aba" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019e22b2-1a2c-769f-b41d-eb74a6fde4c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_019e22b2-1a2c-7889-8d34-1b9ff7f67daf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7447-a981-9fd927a55aba" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_019e22b2-1a2c-7889-8d34-1b9ff7f67daf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019e22b2-1a2c-7c41-8580-d2a1e75f822e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7447-a981-9fd927a55aba" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019e22b2-1a2c-7c41-8580-d2a1e75f822e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019e22b2-1a2c-70de-a88b-26628d96d8e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7447-a981-9fd927a55aba" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019e22b2-1a2c-70de-a88b-26628d96d8e0" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/PropertyandEquipmentNetNarrativeDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#PropertyandEquipmentNetNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/PropertyandEquipmentNetNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7a0d-99f8-a6de2ed7f81a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e22b2-1a2c-74c3-b612-6f913a3c3b49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7a0d-99f8-a6de2ed7f81a" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e22b2-1a2c-74c3-b612-6f913a3c3b49" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e22b2-1a2c-782a-889c-2684e022d1f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e22b2-1a2c-74c3-b612-6f913a3c3b49" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e22b2-1a2c-782a-889c-2684e022d1f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-782a-889c-2684e022d1f2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e22b2-1a2c-782a-889c-2684e022d1f2" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-782a-889c-2684e022d1f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-7ff2-a42d-055f0fb0f52b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e22b2-1a2c-782a-889c-2684e022d1f2" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-7ff2-a42d-055f0fb0f52b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ScannerAppliancesAndOtherComputerEquipmentSubjectToSubscriptionMember_019e22b2-1a2c-787d-b10b-aad1d3188419" xlink:href="qlys-20260630.xsd#qlys_ScannerAppliancesAndOtherComputerEquipmentSubjectToSubscriptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-7ff2-a42d-055f0fb0f52b" xlink:to="loc_qlys_ScannerAppliancesAndOtherComputerEquipmentSubjectToSubscriptionMember_019e22b2-1a2c-787d-b10b-aad1d3188419" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ScannerAppliancesAndOtherComputerEquipmentNotPlacedInServiceMember_019e22b2-1a2c-7cff-93ff-aa5990263e93" xlink:href="qlys-20260630.xsd#qlys_ScannerAppliancesAndOtherComputerEquipmentNotPlacedInServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e22b2-1a2c-7ff2-a42d-055f0fb0f52b" xlink:to="loc_qlys_ScannerAppliancesAndOtherComputerEquipmentNotPlacedInServiceMember_019e22b2-1a2c-7cff-93ff-aa5990263e93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019e22b2-1a2c-780f-a2e8-65dbcff2a16f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7a0d-99f8-a6de2ed7f81a" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019e22b2-1a2c-780f-a2e8-65dbcff2a16f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_DepreciationDepletionAndAmortizationExcludingIntangibleAssetAmortization_019e22b2-1a2c-76e8-915f-7361ab15e094" xlink:href="qlys-20260630.xsd#qlys_DepreciationDepletionAndAmortizationExcludingIntangibleAssetAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e22b2-1a2c-7a0d-99f8-a6de2ed7f81a" xlink:to="loc_qlys_DepreciationDepletionAndAmortizationExcludingIntangibleAssetAmortization_019e22b2-1a2c-76e8-915f-7361ab15e094" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/RevenuefromContractswithCustomersScheduleofExpectedRevenuefromAllRemainingPerformanceObligationsDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#RevenuefromContractswithCustomersScheduleofExpectedRevenuefromAllRemainingPerformanceObligationsDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/RevenuefromContractswithCustomersScheduleofExpectedRevenuefromAllRemainingPerformanceObligationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-797c-919c-2e235bfac841" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019e22b2-1a2c-74dc-a2e3-da94107ee2d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-797c-919c-2e235bfac841" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019e22b2-1a2c-74dc-a2e3-da94107ee2d2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019e22b2-1a2c-791c-85fb-105d5724d831" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019e22b2-1a2c-74dc-a2e3-da94107ee2d2" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019e22b2-1a2c-791c-85fb-105d5724d831" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019e22b2-1a2c-7202-be85-35b2da46d817" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-797c-919c-2e235bfac841" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019e22b2-1a2c-7202-be85-35b2da46d817" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e22b2-1a2c-766c-b1f0-c9874cb3adf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-797c-919c-2e235bfac841" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e22b2-1a2c-766c-b1f0-c9874cb3adf7" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/RevenuefromContractswithCustomersScheduleofExpectedRevenuefromAllRemainingPerformanceObligationsDetails_1" xlink:type="simple" xlink:href="qlys-20260630.xsd#RevenuefromContractswithCustomersScheduleofExpectedRevenuefromAllRemainingPerformanceObligationsDetails_1"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/RevenuefromContractswithCustomersScheduleofExpectedRevenuefromAllRemainingPerformanceObligationsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-797c-919c-2e235bfac841" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019e22b2-1a2c-74dc-a2e3-da94107ee2d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-797c-919c-2e235bfac841" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019e22b2-1a2c-74dc-a2e3-da94107ee2d2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019e22b2-1a2c-7202-be85-35b2da46d817" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-797c-919c-2e235bfac841" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019e22b2-1a2c-7202-be85-35b2da46d817" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e22b2-1a2c-766c-b1f0-c9874cb3adf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-797c-919c-2e235bfac841" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e22b2-1a2c-766c-b1f0-c9874cb3adf7" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/RevenuefromContractswithCustomersScheduleofRevenuebySalesChannelDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#RevenuefromContractswithCustomersScheduleofRevenuebySalesChannelDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/RevenuefromContractswithCustomersScheduleofRevenuebySalesChannelDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-71c2-8c11-7aac718863f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019e22b2-1a2c-7e14-86d2-b592ade1ca23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-71c2-8c11-7aac718863f0" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019e22b2-1a2c-7e14-86d2-b592ade1ca23" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019e22b2-1a2c-7a9c-afe5-07d178534d4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019e22b2-1a2c-7e14-86d2-b592ade1ca23" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019e22b2-1a2c-7a9c-afe5-07d178534d4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e22b2-1a2c-7a9c-afe5-07d178534d4d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019e22b2-1a2c-7a9c-afe5-07d178534d4d" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e22b2-1a2c-7a9c-afe5-07d178534d4d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e22b2-1a2c-7f92-98f6-0217d72c31d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019e22b2-1a2c-7a9c-afe5-07d178534d4d" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e22b2-1a2c-7f92-98f6-0217d72c31d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesChannelDirectlyToConsumerMember_019e22b2-1a2c-7d69-a0df-80ffe626f140" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SalesChannelDirectlyToConsumerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e22b2-1a2c-7f92-98f6-0217d72c31d1" xlink:to="loc_us-gaap_SalesChannelDirectlyToConsumerMember_019e22b2-1a2c-7d69-a0df-80ffe626f140" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesChannelThroughIntermediaryMember_019e22b2-1a2c-7142-b0d7-7d5b948b5bc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SalesChannelThroughIntermediaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e22b2-1a2c-7f92-98f6-0217d72c31d1" xlink:to="loc_us-gaap_SalesChannelThroughIntermediaryMember_019e22b2-1a2c-7142-b0d7-7d5b948b5bc5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e22b2-1a2c-7a56-927c-a2150b8dfd40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019e22b2-1a2c-71c2-8c11-7aac718863f0" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e22b2-1a2c-7a56-927c-a2150b8dfd40" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/IntangibleAssetsNetScheduleofCarryingValueofIntangibleAssetsDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#IntangibleAssetsNetScheduleofCarryingValueofIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/IntangibleAssetsNetScheduleofCarryingValueofIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e22b2-1a2c-7968-bc6c-58fa50e9ba53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e22b2-1a2c-73a1-a69e-c495a5dcad3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e22b2-1a2c-7968-bc6c-58fa50e9ba53" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e22b2-1a2c-73a1-a69e-c495a5dcad3c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e22b2-1a2c-7216-a578-379653aa04da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019e22b2-1a2c-73a1-a69e-c495a5dcad3c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e22b2-1a2c-7216-a578-379653aa04da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e22b2-1a2c-7216-a578-379653aa04da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e22b2-1a2c-7216-a578-379653aa04da" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e22b2-1a2c-7216-a578-379653aa04da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e22b2-1a2c-7cc4-b987-9dc68a2e5c28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e22b2-1a2c-7216-a578-379653aa04da" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e22b2-1a2c-7cc4-b987-9dc68a2e5c28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_019e22b2-1a2c-76f0-92fe-e4c3fb8f8091" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e22b2-1a2c-7cc4-b987-9dc68a2e5c28" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_019e22b2-1a2c-76f0-92fe-e4c3fb8f8091" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019e22b2-1a2c-79bd-8373-6f0215a58812" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e22b2-1a2c-7968-bc6c-58fa50e9ba53" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019e22b2-1a2c-79bd-8373-6f0215a58812" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e22b2-1a2c-729d-8c2f-54d2bb3195d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e22b2-1a2c-7968-bc6c-58fa50e9ba53" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e22b2-1a2c-729d-8c2f-54d2bb3195d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e22b2-1a2c-7867-a6e8-26593ab77d12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e22b2-1a2c-7968-bc6c-58fa50e9ba53" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e22b2-1a2c-7867-a6e8-26593ab77d12" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e22b2-1a2c-7761-b968-1d7b6de53540" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e22b2-1a2c-7968-bc6c-58fa50e9ba53" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e22b2-1a2c-7761-b968-1d7b6de53540" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_019e22b2-1a2c-7dfc-ad0e-caf0061a1ed2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e22b2-1a2c-7968-bc6c-58fa50e9ba53" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_019e22b2-1a2c-7dfc-ad0e-caf0061a1ed2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019e22b2-1a2c-7b5d-875d-d47b67832ba9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019e22b2-1a2c-7968-bc6c-58fa50e9ba53" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019e22b2-1a2c-7b5d-875d-d47b67832ba9" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/StockholdersEquityandStockbasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#StockholdersEquityandStockbasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/StockholdersEquityandStockbasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-76b4-92b6-91935d098656" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-76b4-92b6-91935d098656" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019e22b2-1a2c-792a-b2a0-ec9b49c7048e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-76b4-92b6-91935d098656" xlink:to="loc_us-gaap_PlanNameAxis_019e22b2-1a2c-792a-b2a0-ec9b49c7048e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e22b2-1a2c-792a-b2a0-ec9b49c7048e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019e22b2-1a2c-792a-b2a0-ec9b49c7048e" xlink:to="loc_us-gaap_PlanNameDomain_019e22b2-1a2c-792a-b2a0-ec9b49c7048e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e22b2-1a2c-7fd6-aae4-9c60a60bca15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019e22b2-1a2c-792a-b2a0-ec9b49c7048e" xlink:to="loc_us-gaap_PlanNameDomain_019e22b2-1a2c-7fd6-aae4-9c60a60bca15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_The2012EquityIncentivePlanMember_019e22b2-1a2c-7924-b14d-277a9a7a75b3" xlink:href="qlys-20260630.xsd#qlys_The2012EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e22b2-1a2c-7fd6-aae4-9c60a60bca15" xlink:to="loc_qlys_The2012EquityIncentivePlanMember_019e22b2-1a2c-7924-b14d-277a9a7a75b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_The2021EmployeeStockPurchasePlanMember_019e22b2-1a2c-77a5-828b-53ed15b68574" xlink:href="qlys-20260630.xsd#qlys_The2021EmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e22b2-1a2c-7fd6-aae4-9c60a60bca15" xlink:to="loc_qlys_The2021EmployeeStockPurchasePlanMember_019e22b2-1a2c-77a5-828b-53ed15b68574" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e22b2-1a2c-735d-aa52-090b173e67fa" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-76b4-92b6-91935d098656" xlink:to="loc_srt_RangeAxis_019e22b2-1a2c-735d-aa52-090b173e67fa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e22b2-1a2c-735d-aa52-090b173e67fa_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e22b2-1a2c-735d-aa52-090b173e67fa" xlink:to="loc_srt_RangeMember_019e22b2-1a2c-735d-aa52-090b173e67fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e22b2-1a2c-718b-b435-3a81118883a9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e22b2-1a2c-735d-aa52-090b173e67fa" xlink:to="loc_srt_RangeMember_019e22b2-1a2c-718b-b435-3a81118883a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e22b2-1a2c-75b9-a012-1f6105f4f720" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e22b2-1a2c-718b-b435-3a81118883a9" xlink:to="loc_srt_MinimumMember_019e22b2-1a2c-75b9-a012-1f6105f4f720" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e22b2-1a2c-7a1d-b40c-708ffa0a6012" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e22b2-1a2c-718b-b435-3a81118883a9" xlink:to="loc_srt_MaximumMember_019e22b2-1a2c-7a1d-b40c-708ffa0a6012" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019e22b2-1a2c-746f-902c-954910b062a1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-76b4-92b6-91935d098656" xlink:to="loc_srt_TitleOfIndividualAxis_019e22b2-1a2c-746f-902c-954910b062a1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-746f-902c-954910b062a1_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019e22b2-1a2c-746f-902c-954910b062a1" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-746f-902c-954910b062a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-7a14-8b32-f95c0ce5a364" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019e22b2-1a2c-746f-902c-954910b062a1" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-7a14-8b32-f95c0ce5a364" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ExecutiveOfficerMember_019e22b2-1a2c-7fed-afc6-d8e28c9d21ca" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-7a14-8b32-f95c0ce5a364" xlink:to="loc_srt_ExecutiveOfficerMember_019e22b2-1a2c-7fed-afc6-d8e28c9d21ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e22b2-1a2c-7c69-9c6a-4098de841c7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-76b4-92b6-91935d098656" xlink:to="loc_us-gaap_AwardTypeAxis_019e22b2-1a2c-7c69-9c6a-4098de841c7f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7c69-9c6a-4098de841c7f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e22b2-1a2c-7c69-9c6a-4098de841c7f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7c69-9c6a-4098de841c7f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7179-9b17-592c50e7f9a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e22b2-1a2c-7c69-9c6a-4098de841c7f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7179-9b17-592c50e7f9a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_PerformanceBasedRestrictedStockUnitsMember_019e22b2-1a2c-7d97-b2f4-c9ad58efb628" xlink:href="qlys-20260630.xsd#qlys_PerformanceBasedRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7179-9b17-592c50e7f9a7" xlink:to="loc_qlys_PerformanceBasedRestrictedStockUnitsMember_019e22b2-1a2c-7d97-b2f4-c9ad58efb628" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e22b2-1a2c-794b-b34e-bfa33c95fc60" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7179-9b17-592c50e7f9a7" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e22b2-1a2c-794b-b34e-bfa33c95fc60" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e22b2-1a2c-78d0-8fe1-14c0e8a25acf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7179-9b17-592c50e7f9a7" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e22b2-1a2c-78d0-8fe1-14c0e8a25acf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ESPPSharesMember_019e22b2-1a2c-78e7-b80d-6b643c4baeba" xlink:href="qlys-20260630.xsd#qlys_ESPPSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7179-9b17-592c50e7f9a7" xlink:to="loc_qlys_ESPPSharesMember_019e22b2-1a2c-78e7-b80d-6b643c4baeba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramAxis_019e22b2-1a2c-71bb-9ce7-d666d45328b8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-76b4-92b6-91935d098656" xlink:to="loc_srt_ShareRepurchaseProgramAxis_019e22b2-1a2c-71bb-9ce7-d666d45328b8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019e22b2-1a2c-71bb-9ce7-d666d45328b8_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019e22b2-1a2c-71bb-9ce7-d666d45328b8" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019e22b2-1a2c-71bb-9ce7-d666d45328b8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019e22b2-1a2c-7f39-956a-e437db0f6532" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019e22b2-1a2c-71bb-9ce7-d666d45328b8" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019e22b2-1a2c-7f39-956a-e437db0f6532" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ShareRepurchaseProgramMember_019e22b2-1a2c-7362-94b2-82c5406c4b5f" xlink:href="qlys-20260630.xsd#qlys_ShareRepurchaseProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019e22b2-1a2c-7f39-956a-e437db0f6532" xlink:to="loc_qlys_ShareRepurchaseProgramMember_019e22b2-1a2c-7362-94b2-82c5406c4b5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019e22b2-1a2c-7eb4-9123-281f6fef3e99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019e22b2-1a2c-7eb4-9123-281f6fef3e99" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019e22b2-1a2c-736f-b2cb-fd92559e777b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019e22b2-1a2c-736f-b2cb-fd92559e777b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019e22b2-1a2c-7305-a801-f28a713923b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019e22b2-1a2c-7305-a801-f28a713923b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriod_019e22b2-1a2c-7bb6-af05-5d00d9682fe6" xlink:href="qlys-20260630.xsd#qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriod_019e22b2-1a2c-7bb6-af05-5d00d9682fe6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_EmployeeStockPurchasePlanPercentageOfEligibleCompensationContributable_019e22b2-1a2c-7379-998f-5c944ca1c3ae" xlink:href="qlys-20260630.xsd#qlys_EmployeeStockPurchasePlanPercentageOfEligibleCompensationContributable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_qlys_EmployeeStockPurchasePlanPercentageOfEligibleCompensationContributable_019e22b2-1a2c-7379-998f-5c944ca1c3ae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_019e22b2-1a2c-7f46-9d87-d200a277fa4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_019e22b2-1a2c-7f46-9d87-d200a277fa4f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e22b2-1a2c-7295-8d39-639e6e679aeb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e22b2-1a2c-7295-8d39-639e6e679aeb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e22b2-1a2c-72a2-bdcb-025944fda5b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e22b2-1a2c-72a2-bdcb-025944fda5b6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019e22b2-1a2c-7293-9e6b-28e56da4335e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019e22b2-1a2c-7293-9e6b-28e56da4335e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingNumberOfInstallments_019e22b2-1a2c-75a4-9d2e-182da1be2a1d" xlink:href="qlys-20260630.xsd#qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingNumberOfInstallments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingNumberOfInstallments_019e22b2-1a2c-75a4-9d2e-182da1be2a1d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetPercentage_019e22b2-1a2c-7c96-8530-ef94b5d24815" xlink:href="qlys-20260630.xsd#qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetPercentage_019e22b2-1a2c-7c96-8530-ef94b5d24815" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ShareBasedCompensationPercentageOfSharesThatWillVestUponTrigger_019e22b2-1a2c-7d0f-b003-903e37314682" xlink:href="qlys-20260630.xsd#qlys_ShareBasedCompensationPercentageOfSharesThatWillVestUponTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_qlys_ShareBasedCompensationPercentageOfSharesThatWillVestUponTrigger_019e22b2-1a2c-7d0f-b003-903e37314682" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e22b2-1a2c-7f71-85af-18d719ba1f2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e22b2-1a2c-7f71-85af-18d719ba1f2a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e22b2-1a2c-7786-aa7d-09f0a3ed1133" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e22b2-1a2c-7786-aa7d-09f0a3ed1133" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e22b2-1a2c-7fe0-8d90-a57907b39fef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e22b2-1a2c-7fe0-8d90-a57907b39fef" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e22b2-1a2c-7fc2-8b15-07dc9309a339" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e22b2-1a2c-7fc2-8b15-07dc9309a339" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019e22b2-1a2c-77a1-9263-99dc28006902" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019e22b2-1a2c-77a1-9263-99dc28006902" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019e22b2-1a2c-73fe-b8bc-0806b1944746" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019e22b2-1a2c-73fe-b8bc-0806b1944746" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramExciseTax_019eb107-3db3-79ca-bc8c-fc097c2f7ef6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareRepurchaseProgramExciseTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_ShareRepurchaseProgramExciseTax_019eb107-3db3-79ca-bc8c-fc097c2f7ef6" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramExciseTaxPayable_019e22b2-1a2c-73d3-828a-889732d1d374" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareRepurchaseProgramExciseTaxPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-721d-b49d-7d05d90fb50d" xlink:to="loc_us-gaap_ShareRepurchaseProgramExciseTaxPayable_019e22b2-1a2c-73d3-828a-889732d1d374" xlink:type="arc" order="19"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/StockholdersEquityandStockbasedCompensationScheduleofRestrictedStockUnitActivityDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#StockholdersEquityandStockbasedCompensationScheduleofRestrictedStockUnitActivityDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/StockholdersEquityandStockbasedCompensationScheduleofRestrictedStockUnitActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-7039-b41a-c7cf33f4200b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-74b7-b776-61ad7d16a42a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-7039-b41a-c7cf33f4200b" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-74b7-b776-61ad7d16a42a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e22b2-1a2c-7e23-a9c6-d43d8176e328" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-74b7-b776-61ad7d16a42a" xlink:to="loc_us-gaap_AwardTypeAxis_019e22b2-1a2c-7e23-a9c6-d43d8176e328" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7e23-a9c6-d43d8176e328_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e22b2-1a2c-7e23-a9c6-d43d8176e328" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-7e23-a9c6-d43d8176e328_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-726f-8e43-b78707de0804" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e22b2-1a2c-7e23-a9c6-d43d8176e328" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-726f-8e43-b78707de0804" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e22b2-1a2c-7c8d-a27e-460895929750" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-726f-8e43-b78707de0804" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e22b2-1a2c-7c8d-a27e-460895929750" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_PerformanceBasedRestrictedStockUnitsMember_019e22b2-1a2c-718d-ad99-1914f53f4027" xlink:href="qlys-20260630.xsd#qlys_PerformanceBasedRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e22b2-1a2c-726f-8e43-b78707de0804" xlink:to="loc_qlys_PerformanceBasedRestrictedStockUnitsMember_019e22b2-1a2c-718d-ad99-1914f53f4027" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019e22b2-1a2c-7ff6-8f4c-8bae11a0b1e8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-74b7-b776-61ad7d16a42a" xlink:to="loc_srt_TitleOfIndividualAxis_019e22b2-1a2c-7ff6-8f4c-8bae11a0b1e8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-7ff6-8f4c-8bae11a0b1e8_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019e22b2-1a2c-7ff6-8f4c-8bae11a0b1e8" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-7ff6-8f4c-8bae11a0b1e8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-73d7-b9d6-212532d56d3b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019e22b2-1a2c-7ff6-8f4c-8bae11a0b1e8" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-73d7-b9d6-212532d56d3b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ExecutiveOfficer1Member_019e22b2-1a2c-7345-acfc-283ff63800f6" xlink:href="qlys-20260630.xsd#qlys_ExecutiveOfficer1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-73d7-b9d6-212532d56d3b" xlink:to="loc_qlys_ExecutiveOfficer1Member_019e22b2-1a2c-7345-acfc-283ff63800f6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ExecutiveOfficerMember_019e22b2-1a2c-70a7-84f5-53e2c0d5a9b7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-73d7-b9d6-212532d56d3b" xlink:to="loc_srt_ExecutiveOfficerMember_019e22b2-1a2c-70a7-84f5-53e2c0d5a9b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ExecutiveOfficer2Member_019e22b2-1a2c-7048-8dd9-2a73a741a81d" xlink:href="qlys-20260630.xsd#qlys_ExecutiveOfficer2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-73d7-b9d6-212532d56d3b" xlink:to="loc_qlys_ExecutiveOfficer2Member_019e22b2-1a2c-7048-8dd9-2a73a741a81d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ExecutiveOfficer3Member_019e22b2-1a2c-7de3-bcd6-12c7c34bf6a0" xlink:href="qlys-20260630.xsd#qlys_ExecutiveOfficer3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-73d7-b9d6-212532d56d3b" xlink:to="loc_qlys_ExecutiveOfficer3Member_019e22b2-1a2c-7de3-bcd6-12c7c34bf6a0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ExecutiveOfficer4Member_019e22b2-1a2c-7194-b195-57111d5d1a34" xlink:href="qlys-20260630.xsd#qlys_ExecutiveOfficer4Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e22b2-1a2c-73d7-b9d6-212532d56d3b" xlink:to="loc_qlys_ExecutiveOfficer4Member_019e22b2-1a2c-7194-b195-57111d5d1a34" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e22b2-1a2c-7bf8-8e57-42b7c5cae58c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-7039-b41a-c7cf33f4200b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e22b2-1a2c-7bf8-8e57-42b7c5cae58c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e22b2-1a2c-7b88-83eb-74ca6fd860d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e22b2-1a2c-7bf8-8e57-42b7c5cae58c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e22b2-1a2c-7b88-83eb-74ca6fd860d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e22b2-1a2c-7ee7-b0a4-0577c9ed8286" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e22b2-1a2c-7bf8-8e57-42b7c5cae58c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e22b2-1a2c-7ee7-b0a4-0577c9ed8286" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e22b2-1a2c-7f30-815a-f038ab2ec088" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e22b2-1a2c-7bf8-8e57-42b7c5cae58c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e22b2-1a2c-7f30-815a-f038ab2ec088" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e22b2-1a2c-7969-9e3e-0f6d31c1de40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e22b2-1a2c-7bf8-8e57-42b7c5cae58c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e22b2-1a2c-7969-9e3e-0f6d31c1de40" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e22b2-1a2c-7bbe-b404-ec4eb771827e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestNumberOfShares_019e22b2-1a2c-7cd4-83b9-32a57a43885d" xlink:href="qlys-20260630.xsd#qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestNumberOfShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-7039-b41a-c7cf33f4200b" xlink:to="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestNumberOfShares_019e22b2-1a2c-7cd4-83b9-32a57a43885d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e22b2-1a2c-7778-b46f-03986549b611" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-7039-b41a-c7cf33f4200b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e22b2-1a2c-7778-b46f-03986549b611" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e22b2-1a2c-7970-867a-30920b8ee267" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e22b2-1a2c-7778-b46f-03986549b611" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e22b2-1a2c-7970-867a-30920b8ee267" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e22b2-1a2c-7f67-aa4f-3ef6e8c29509" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e22b2-1a2c-7778-b46f-03986549b611" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e22b2-1a2c-7f67-aa4f-3ef6e8c29509" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e22b2-1a2c-7aba-a190-0c7fc43e3aea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e22b2-1a2c-7778-b46f-03986549b611" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e22b2-1a2c-7aba-a190-0c7fc43e3aea" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e22b2-1a2c-7025-9d2a-93233e5aa119" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e22b2-1a2c-7778-b46f-03986549b611" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e22b2-1a2c-7025-9d2a-93233e5aa119" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e22b2-1a2c-7b74-9f4f-6054ad73bf31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestWeightedAverageGrantDateFairValue_019e22b2-1a2c-7598-b139-9a521b117248" xlink:href="qlys-20260630.xsd#qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-7039-b41a-c7cf33f4200b" xlink:to="loc_qlys_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestWeightedAverageGrantDateFairValue_019e22b2-1a2c-7598-b139-9a521b117248" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/StockholdersEquityandStockbasedCompensationScheduleofStockbasedCompensationExpenseDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#StockholdersEquityandStockbasedCompensationScheduleofStockbasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/StockholdersEquityandStockbasedCompensationScheduleofStockbasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-701d-b265-52eade7c69b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-70c1-9e2c-bbaa8bf85efd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-701d-b265-52eade7c69b3" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-70c1-9e2c-bbaa8bf85efd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e22b2-1a2c-741c-a9a7-927c4286d08b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-70c1-9e2c-bbaa8bf85efd" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e22b2-1a2c-741c-a9a7-927c4286d08b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e22b2-1a2c-7fe6-ae85-13d6414f040b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-701d-b265-52eade7c69b3" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e22b2-1a2c-7fe6-ae85-13d6414f040b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/StockholdersEquityandStockbasedCompensationScheduleofStockbasedCompensationExpenseDetails_1" xlink:type="simple" xlink:href="qlys-20260630.xsd#StockholdersEquityandStockbasedCompensationScheduleofStockbasedCompensationExpenseDetails_1"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/StockholdersEquityandStockbasedCompensationScheduleofStockbasedCompensationExpenseDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-701d-b265-52eade7c69b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-70c1-9e2c-bbaa8bf85efd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-701d-b265-52eade7c69b3" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e22b2-1a2c-70c1-9e2c-bbaa8bf85efd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e22b2-1a2c-7fe6-ae85-13d6414f040b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e22b2-1a2c-701d-b265-52eade7c69b3" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e22b2-1a2c-7fe6-ae85-13d6414f040b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/SegmentandGeographicAreaInformationScheduleofReportedSegmentRevenueSegmentProfitOrLossDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#SegmentandGeographicAreaInformationScheduleofReportedSegmentRevenueSegmentProfitOrLossDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/SegmentandGeographicAreaInformationScheduleofReportedSegmentRevenueSegmentProfitOrLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e22b2-1a2c-7742-829a-8cf3ac18d4c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e22b2-1a2c-7742-829a-8cf3ac18d4c7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e22b2-1a2c-72d2-b17b-f56c6dae1b9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e22b2-1a2c-7742-829a-8cf3ac18d4c7" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e22b2-1a2c-72d2-b17b-f56c6dae1b9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e22b2-1a2c-72d2-b17b-f56c6dae1b9e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e22b2-1a2c-72d2-b17b-f56c6dae1b9e" xlink:to="loc_us-gaap_SegmentDomain_019e22b2-1a2c-72d2-b17b-f56c6dae1b9e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e22b2-1a2c-7071-84ba-9736546ddfe1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e22b2-1a2c-72d2-b17b-f56c6dae1b9e" xlink:to="loc_us-gaap_SegmentDomain_019e22b2-1a2c-7071-84ba-9736546ddfe1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_ReportableSegmentMember_019e22b2-1a2c-76ba-bdb7-7135e376437e" xlink:href="qlys-20260630.xsd#qlys_ReportableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e22b2-1a2c-7071-84ba-9736546ddfe1" xlink:to="loc_qlys_ReportableSegmentMember_019e22b2-1a2c-76ba-bdb7-7135e376437e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e22b2-1a2c-7178-92bd-d974f1cc9b02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e22b2-1a2c-7178-92bd-d974f1cc9b02" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationRevenueAbstract_019e22b2-1a2c-715d-ad46-f4936f492af4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationRevenueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_SegmentReportingInformationRevenueAbstract_019e22b2-1a2c-715d-ad46-f4936f492af4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_019e22b2-1a2c-7593-98f3-3c6e0d84a7d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationRevenueAbstract_019e22b2-1a2c-715d-ad46-f4936f492af4" xlink:to="loc_us-gaap_CostOfRevenue_019e22b2-1a2c-7593-98f3-3c6e0d84a7d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_019e22b2-1a2c-77b0-916f-e780264313a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationRevenueAbstract_019e22b2-1a2c-715d-ad46-f4936f492af4" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_019e22b2-1a2c-77b0-916f-e780264313a9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpense_019e22b2-1a2c-77c3-b84e-93564abcd132" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingAndMarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationRevenueAbstract_019e22b2-1a2c-715d-ad46-f4936f492af4" xlink:to="loc_us-gaap_SellingAndMarketingExpense_019e22b2-1a2c-77c3-b84e-93564abcd132" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_019e22b2-1a2c-760b-8572-81116b4fc7a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationRevenueAbstract_019e22b2-1a2c-715d-ad46-f4936f492af4" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_019e22b2-1a2c-760b-8572-81116b4fc7a2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingOtherItemAmount_019e22b2-1a2c-79a5-85fe-87adba9bc690" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingOtherItemAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_SegmentReportingOtherItemAmount_019e22b2-1a2c-79a5-85fe-87adba9bc690" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019e22b2-1a2c-715d-bbe1-55abca78df7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_ShareBasedCompensation_019e22b2-1a2c-715d-bbe1-55abca78df7b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_019e22b2-1a2c-7b90-9ebe-cb55d695f12c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_019e22b2-1a2c-7b90-9ebe-cb55d695f12c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterest_019e22b2-1a2c-7a47-b1c9-00efdf573cfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_InvestmentIncomeInterest_019e22b2-1a2c-7a47-b1c9-00efdf573cfd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019e22b2-1a2c-7f6b-bd59-3df47912a345" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019e22b2-1a2c-7f6b-bd59-3df47912a345" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e22b2-1a2c-78d3-870b-cfef4b187fe0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e22b2-1a2c-751c-92e2-aeeeb119c1d3" xlink:to="loc_us-gaap_NetIncomeLoss_019e22b2-1a2c-78d3-870b-cfef4b187fe0" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/SegmentandGeographicAreaInformationScheduleofRevenueandPropertyandEquipmentNetbyGeographicAreaDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#SegmentandGeographicAreaInformationScheduleofRevenueandPropertyandEquipmentNetbyGeographicAreaDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/SegmentandGeographicAreaInformationScheduleofRevenueandPropertyandEquipmentNetbyGeographicAreaDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e22b2-1a2c-7fe4-94a5-1e1e05ec7177" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e22b2-1a2c-7543-b51d-239e8feb3591" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e22b2-1a2c-7fe4-94a5-1e1e05ec7177" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e22b2-1a2c-7543-b51d-239e8feb3591" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019e22b2-1a2c-7e61-a3b9-46502351b18a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e22b2-1a2c-7543-b51d-239e8feb3591" xlink:to="loc_srt_StatementGeographicalAxis_019e22b2-1a2c-7e61-a3b9-46502351b18a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e22b2-1a2c-7e61-a3b9-46502351b18a_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019e22b2-1a2c-7e61-a3b9-46502351b18a" xlink:to="loc_srt_SegmentGeographicalDomain_019e22b2-1a2c-7e61-a3b9-46502351b18a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e22b2-1a2c-7c97-9c7e-3c1462fa3cd0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019e22b2-1a2c-7e61-a3b9-46502351b18a" xlink:to="loc_srt_SegmentGeographicalDomain_019e22b2-1a2c-7c97-9c7e-3c1462fa3cd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019e22b2-1a2c-7da9-9fac-a730a8382bc7" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e22b2-1a2c-7c97-9c7e-3c1462fa3cd0" xlink:to="loc_country_US_019e22b2-1a2c-7da9-9fac-a730a8382bc7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_019e22b2-1a2c-7b50-998d-6a3d4c3e7a09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e22b2-1a2c-7c97-9c7e-3c1462fa3cd0" xlink:to="loc_us-gaap_NonUsMember_019e22b2-1a2c-7b50-998d-6a3d4c3e7a09" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IN_019e22b2-1a2c-70d7-b711-c8b9b5c4a205" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_IN"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e22b2-1a2c-7c97-9c7e-3c1462fa3cd0" xlink:to="loc_country_IN_019e22b2-1a2c-70d7-b711-c8b9b5c4a205" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_OtherGeographicAreasMember_019e22b2-1a2c-7335-967e-d82e4254c399" xlink:href="qlys-20260630.xsd#qlys_OtherGeographicAreasMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e22b2-1a2c-7c97-9c7e-3c1462fa3cd0" xlink:to="loc_qlys_OtherGeographicAreasMember_019e22b2-1a2c-7335-967e-d82e4254c399" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e22b2-1a2c-78fd-a318-1f8f74e8cdb5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e22b2-1a2c-7fe4-94a5-1e1e05ec7177" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e22b2-1a2c-78fd-a318-1f8f74e8cdb5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_019e22b2-1a2c-78a8-b90e-26a4159346e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e22b2-1a2c-7fe4-94a5-1e1e05ec7177" xlink:to="loc_us-gaap_NoncurrentAssets_019e22b2-1a2c-78a8-b90e-26a4159346e0" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.qualys.com/role/NetIncomePerShareScheduleofAntidilutiveNetIncomePerShareDetails" xlink:type="simple" xlink:href="qlys-20260630.xsd#NetIncomePerShareScheduleofAntidilutiveNetIncomePerShareDetails"/>
  <link:definitionLink xlink:role="http://www.qualys.com/role/NetIncomePerShareScheduleofAntidilutiveNetIncomePerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e22b2-1a2c-7568-a591-eef75ef0dbea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e22b2-1a2c-738f-9e8e-9ff192997ef0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e22b2-1a2c-7568-a591-eef75ef0dbea" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e22b2-1a2c-738f-9e8e-9ff192997ef0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e22b2-1a2c-7719-ada3-a06886b17119" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e22b2-1a2c-738f-9e8e-9ff192997ef0" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e22b2-1a2c-7719-ada3-a06886b17119" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e22b2-1a2c-7719-ada3-a06886b17119_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e22b2-1a2c-7719-ada3-a06886b17119" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e22b2-1a2c-7719-ada3-a06886b17119_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e22b2-1a2c-7aed-a543-b7b7654cfad4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e22b2-1a2c-7719-ada3-a06886b17119" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e22b2-1a2c-7aed-a543-b7b7654cfad4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e22b2-1a2c-7125-a79c-ca90032e78e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e22b2-1a2c-7aed-a543-b7b7654cfad4" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e22b2-1a2c-7125-a79c-ca90032e78e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e22b2-1a2c-716a-8426-eb55dc114013" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e22b2-1a2c-7aed-a543-b7b7654cfad4" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e22b2-1a2c-716a-8426-eb55dc114013" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_qlys_EmployeeStockPurchasePlanSharesMember_019e22b2-1a2c-7776-b260-d6214b86ef9f" xlink:href="qlys-20260630.xsd#qlys_EmployeeStockPurchasePlanSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e22b2-1a2c-7aed-a543-b7b7654cfad4" xlink:to="loc_qlys_EmployeeStockPurchasePlanSharesMember_019e22b2-1a2c-7776-b260-d6214b86ef9f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e22b2-1a2c-7806-a038-150472bba872" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e22b2-1a2c-7568-a591-eef75ef0dbea" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e22b2-1a2c-7806-a038-150472bba872" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
