<SEC-DOCUMENT>0001644956-25-000052.txt : 20251112
<SEC-HEADER>0001644956-25-000052.hdr.sgml : 20251112
<ACCEPTANCE-DATETIME>20251112182426
ACCESSION NUMBER:		0001644956-25-000052
CONFORMED SUBMISSION TYPE:	SCHEDULE 13G/A
PUBLIC DOCUMENT COUNT:		1
FILED AS OF DATE:		20251112
DATE AS OF CHANGE:		20251112

SUBJECT COMPANY:	

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			MERCURY SYSTEMS INC
		CENTRAL INDEX KEY:			0001049521
		STANDARD INDUSTRIAL CLASSIFICATION:	ELECTRONIC COMPONENTS & ACCESSORIES [3670]
		ORGANIZATION NAME:           	04 Manufacturing
		EIN:				042741391
		STATE OF INCORPORATION:			MA
		FISCAL YEAR END:			0630

	FILING VALUES:
		FORM TYPE:		SCHEDULE 13G/A
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	005-52823
		FILM NUMBER:		251474712

	BUSINESS ADDRESS:	
		STREET 1:		50 MINUTEMAN ROAD
		CITY:			ANDOVER
		STATE:			MA
		ZIP:			01810
		BUSINESS PHONE:		9782561300

	MAIL ADDRESS:	
		STREET 1:		50 MINUTEMAN ROAD
		CITY:			ANDOVER
		STATE:			MA
		ZIP:			01810

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	MERCURY COMPUTER SYSTEMS INC
		DATE OF NAME CHANGE:	19971112

FILED BY:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			WILLIAM BLAIR INVESTMENT MANAGEMENT, LLC
		CENTRAL INDEX KEY:			0001644956
		ORGANIZATION NAME:           	
		EIN:				472614791
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		SCHEDULE 13G/A

	BUSINESS ADDRESS:	
		STREET 1:		150 NORTH RIVERSIDE PLAZA
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60606
		BUSINESS PHONE:		312-236-1600

	MAIL ADDRESS:	
		STREET 1:		150 NORTH RIVERSIDE PLAZA
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60606
</SEC-HEADER>
<DOCUMENT>
<TYPE>SCHEDULE 13G/A
<SEQUENCE>1
<FILENAME>primary_doc.xml
<TEXT>
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  <headerData>
    <submissionType>SCHEDULE 13G/A</submissionType>
    <previousAccessionNumber>0001104659-21-020608</previousAccessionNumber>
    <filerInfo>
      <filer>
        <filerCredentials>
          <cik>0001644956</cik>
          <ccc>XXXXXXXX</ccc>
        </filerCredentials>
      </filer>
      <liveTestFlag>LIVE</liveTestFlag>



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  </headerData>
  <formData>
    <coverPageHeader>
      <amendmentNo>9</amendmentNo>
      <securitiesClassTitle>Common Stock</securitiesClassTitle>
      <eventDateRequiresFilingThisStatement>09/30/2025</eventDateRequiresFilingThisStatement>
      <issuerInfo>
        <issuerCik>0001049521</issuerCik>
        <issuerName>MERCURY SYSTEMS INC</issuerName>
        <issuerCusip>589378108</issuerCusip>
        <issuerPrincipalExecutiveOfficeAddress>
          <com:street1>50 MINUTEMAN ROAD</com:street1>
          <com:street2>50 MINUTEMAN ROAD</com:street2>
          <com:city>ANDOVER</com:city>
          <com:stateOrCountry>MA</com:stateOrCountry>
          <com:zipCode>01810</com:zipCode>
        </issuerPrincipalExecutiveOfficeAddress>
      </issuerInfo>
      <designateRulesPursuantThisScheduleFiled>
        <designateRulePursuantThisScheduleFiled>Rule 13d-1(b)</designateRulePursuantThisScheduleFiled>
      </designateRulesPursuantThisScheduleFiled>
    </coverPageHeader>
    <coverPageHeaderReportingPersonDetails>

      <reportingPersonName>WILLIAM BLAIR INVESTMENT MANAGEMENT, LLC</reportingPersonName>
      <citizenshipOrOrganization>DE</citizenshipOrOrganization>
      <reportingPersonBeneficiallyOwnedNumberOfShares>
        <soleVotingPower>2252300.00</soleVotingPower>
        <sharedVotingPower>0.00</sharedVotingPower>
        <soleDispositivePower>2526133.00</soleDispositivePower>
        <sharedDispositivePower>0.00</sharedDispositivePower>
      </reportingPersonBeneficiallyOwnedNumberOfShares>
      <reportingPersonBeneficiallyOwnedAggregateNumberOfShares>2526133.00</reportingPersonBeneficiallyOwnedAggregateNumberOfShares>
      <classPercent>4.2</classPercent>
      <typeOfReportingPerson>IA</typeOfReportingPerson>
    </coverPageHeaderReportingPersonDetails>
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      <item1>
        <issuerName>MERCURY SYSTEMS INC</issuerName>
        <issuerPrincipalExecutiveOfficeAddress>50 MINUTEMAN ROAD, 50 MINUTEMAN ROAD, ANDOVER, MASSACHUSETTS, 01810.</issuerPrincipalExecutiveOfficeAddress>
      </item1>
      <item2>
        <filingPersonName>William Blair Investment Management, LLC</filingPersonName>
        <principalBusinessOfficeOrResidenceAddress>150 North Riverside Plaza, Chicago, IL 60606</principalBusinessOfficeOrResidenceAddress>
        <citizenship>Delaware</citizenship>
      </item2>
      <item3>
        <notApplicableFlag>N</notApplicableFlag>
        <typeOfPersonFiling>IA</typeOfPersonFiling>
      </item3>
      <item4>
        <amountBeneficiallyOwned>2,526,133</amountBeneficiallyOwned>
        <classPercent>4.2%</classPercent>
        <numberOfSharesPersonHas>
          <solePowerOrDirectToVote>2,252,300</solePowerOrDirectToVote>
          <sharedPowerOrDirectToVote>0</sharedPowerOrDirectToVote>
          <solePowerOrDirectToDispose>2,526,133</solePowerOrDirectToDispose>
          <sharedPowerOrDirectToDispose>0</sharedPowerOrDirectToDispose>
        </numberOfSharesPersonHas>
      </item4>
      <item5>
        <classOwnership5PercentOrLess>Y</classOwnership5PercentOrLess>
      </item5>
      <item6>
        <notApplicableFlag>Y</notApplicableFlag>
      </item6>
      <item7>
        <notApplicableFlag>Y</notApplicableFlag>
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      <item8>
        <notApplicableFlag>Y</notApplicableFlag>
      </item8>
      <item9>
        <notApplicableFlag>Y</notApplicableFlag>
      </item9>
      <item10>
        <notApplicableFlag>N</notApplicableFlag>
        <certifications>By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under &amp;#167; 240.14a-11.

</certifications>
      </item10>
    </items>
    <signatureInformation>
      <reportingPersonName>WILLIAM BLAIR INVESTMENT MANAGEMENT, LLC</reportingPersonName>
      <signatureDetails>
        <signature>Walter R. Randall, Jr.</signature>
        <title>Chief Compliance Officer</title>
        <date>11/12/2025</date>
      </signatureDetails>
    </signatureInformation>
  </formData>
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</SEC-DOCUMENT>
