<?xml version="1.0"?>

<ownershipDocument>

    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2003-10-31</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000730000</issuerCik>
        <issuerName>SUPERTEX INC</issuerName>
        <issuerTradingSymbol>supx</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001197204</rptOwnerCik>
            <rptOwnerName>SCHIE DAVID</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1></rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity></rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode></rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>V.P Telecom &amp; Broadband Prod.</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>

        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2003-10-31</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>204</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>13.5915</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>204</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>

    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Shares Purchased under Supertex Inc., 2000 Employee Stock Purchase Plan</footnote>
        <footnote id="F2">Stock Option Grant:  NQ Option (to purchase common stock).  Exercise Price $28.3125.  Shares Granted: 60,000.  Shares currently exercisable: 36,000. 12,000 shares exercisable on 6/1/04.  12,000 shares exercisable on 6/1/05.  Expiration date 6/1/07.  Held Direct.</footnote>
        <footnote id="F3">Stock Option Grant:  ISO Option (to purchase common stock).  Exercise Price $12.5300,  Shares granted: 7,980.  Shares currently exercisable:  0.  7,980 shares exercisable on 6/1/06.  Expiration date: 6/1/2008.  Held Direct.</footnote>
        <footnote id="F4">Stock Option Grant:  NQ Option (to purchase common stock).  Exercise Price $12.5300.  Shares granted 4,020.  Shares exercisable currently.  0.  4,020 shares exercisable on 6/1/06.  Expiration date 6/1/2008.  Held direct.</footnote>
    </footnotes>


    <ownerSignature>
        <signatureName>David Schie</signatureName>
        <signatureDate>2003-11-03</signatureDate>
    </ownerSignature>

</ownershipDocument>

