<SEC-DOCUMENT>0000831641-17-000098.txt : 20171122
<SEC-HEADER>0000831641-17-000098.hdr.sgml : 20171122
<ACCEPTANCE-DATETIME>20171122162351
ACCESSION NUMBER:		0000831641-17-000098
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20171121
FILED AS OF DATE:		20171122
DATE AS OF CHANGE:		20171122

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			LEMMON RICHARD A
		CENTRAL INDEX KEY:			0001187157

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-19655
		FILM NUMBER:		171220176

	MAIL ADDRESS:	
		STREET 1:		3475 EAST FOOTHILL BOULEVARD
		CITY:			PASADENA
		STATE:			CA
		ZIP:			91107

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			TETRA TECH INC
		CENTRAL INDEX KEY:			0000831641
		STANDARD INDUSTRIAL CLASSIFICATION:	SERVICES-ENGINEERING SERVICES [8711]
		IRS NUMBER:				954148514
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			0928

	BUSINESS ADDRESS:	
		STREET 1:		3475 EAST FOOTHILL BOULEVARD
		CITY:			PASADENA
		STATE:			CA
		ZIP:			91107
		BUSINESS PHONE:		6263514664

	MAIL ADDRESS:	
		STREET 1:		3475 EAST FOOTHILL BOULEVARD
		CITY:			PASADENA
		STATE:			CA
		ZIP:			91107
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_151138581727466.xml
<DESCRIPTION>FORM 4
<TEXT>
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    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-11-21</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000831641</issuerCik>
        <issuerName>TETRA TECH INC</issuerName>
        <issuerTradingSymbol>TTEK</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001187157</rptOwnerCik>
            <rptOwnerName>LEMMON RICHARD A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>3475 EAST FOOTHILL BOULEVARD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>PASADENA,</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>91107</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP, CORPORATE ADMINISTRATION</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON STOCK</value>
            </securityTitle>
            <transactionDate>
                <value>2017-11-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>6156</value>
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                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
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                <transactionAcquiredDisposedCode>
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            <securityTitle>
                <value>COMMON STOCK</value>
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                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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                    <footnoteId id="F4"/>
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                    <value>D</value>
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                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Each performance share unit (&quot;PSU&quot;) represents the right to receive one share of Common Stock.  The PSUs awarded cliff-vest after a 3-year performance period.  The Performance Period was the Company's fiscal years 2015, 2016 and 2017.  The number of PSUs earned by the Awardee was determined at the end of the Performance Period by measuring earnings per share (&quot;EPS&quot;) growth, which was  35% year-over-year growth.  Therefore, 200% of the 3,078 PSUs awarded on November 21, 2014 were earned and vested on November 21, 2017.</footnote>
        <footnote id="F2">Each performance share unit (&quot;PSU&quot;) represents the right to receive one share of Common Stock.  The PSUs awarded cliff-vest after a 3-year performance period.  The Performance Period was the Company's fiscal years 2015, 2016 and 2017.  The number of PSUs earned by the Awardee for the Performance Period was determined at the end of the Performance Period by measuring relative total shareholder return (&quot;TSR&quot;), with equal weight given to the selected industry peer group and to the S&amp;P 1000.  The TSR peer group performance percentile was 89%, and the TSR S&amp;P 1000 performance percentile was 82%.  Straight line interpolation determined the vesting credit percentage.  Therefore, 189% of the 3,078 PSUs awarded on November 21, 2014 were earned and vested on November 21, 2017.</footnote>
        <footnote id="F3">4,533 of the shares underlying PSUs which vested on 11/21/2017 were withheld to pay the related tax liability.</footnote>
        <footnote id="F4">402 of the shares underlying restricted stock which vested on 11/21/2017 were withheld to pay the related tax liability.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Janis B. Salin for Richard A. Lemmon</signatureName>
        <signatureDate>2017-11-22</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
