<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType>
<periodOfReport>2003-05-16</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0001161396</issuerCik>
        <issuerName>GENESIS MICROCHIP INC /DE</issuerName>
        <issuerTradingSymbol>GNSS</issuerTradingSymbol></issuer>
<reportingOwner>
<reportingOwnerId>
            <rptOwnerCik>0001231663</rptOwnerCik>
            <rptOwnerName>CHAN TZOYAO</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>Sr. VP. Engineering</officerTitle>
<otherText></otherText></reportingOwnerRelationship></reportingOwner>
<derivativeSecurity>
<securityTitle>
<value>Stock Option (Right to Buy)</value></securityTitle>
<conversionOrExercisePrice>
<value>16.81</value></conversionOrExercisePrice>
<transactionDate>
<value>2003-05-16</value></transactionDate>
<transactionCoding>
<transactionFormType>4</transactionFormType>
<transactionCode>A</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved></transactionCoding>
<transactionAmounts>
<transactionShares>
<value>50000</value></transactionShares>
<transactionValue>
<value>0.00</value></transactionValue>
<transactionAcquiredDisposedCode>
<value>A</value></transactionAcquiredDisposedCode></transactionAmounts>
<exerciseDate>
<value>2004-05-16</value>
<footnoteId id="F1"/></exerciseDate>
<expirationDate>
<value>2013-05-16</value></expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Common Shares</value></underlyingSecurityTitle>
<underlyingSecurityShares>
<value>50000</value></underlyingSecurityShares></underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>214056</value></sharesOwnedFollowingTransaction></postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value></directOrIndirectOwnership>
<natureOfOwnership>
<value></value></natureOfOwnership></ownershipNature></derivativeSecurity>
<footnotes>
<footnote id="F1">25% of the total number of shares become exercisable on this date and an additional 1/48th of the total number of shares become exercisable each month thereafter.</footnote></footnotes>
<ownerSignature>
<signatureName>/s/Ava Hahn, Attorney-in-fact</signatureName>
<signatureDate>2003-06-03</signatureDate></ownerSignature></ownershipDocument>


