<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-04-09</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000107687</issuerCik>
        <issuerName>WINNEBAGO INDUSTRIES INC</issuerName>
        <issuerTradingSymbol>WGO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001382452</rptOwnerCik>
            <rptOwnerName>Happe Michael J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>WINNEBAGO INDUSTRIES, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>P.O. BOX 152</rptOwnerStreet2>
            <rptOwnerCity>FOREST CITY</rptOwnerCity>
            <rptOwnerState>IA</rptOwnerState>
            <rptOwnerZipCode>50436</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President &amp; CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, $.50 par value</value>
            </securityTitle>
            <transactionDate>
                <value>2020-04-09</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>37.5256</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>89626</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Reflects the weighted average price of 3,000 shares of common stock of Winnebago Industries Inc. purchased by the reporting person in multiple transactions on April 9, 2020 with purchase prices ranging from $37.4377 to $37.6282 per share. The reporting person undertakes to provide upon request by the U.S. Securities and Exchange Commission staff, the issuer, or a security holder of the issuer, full information regarding the number of shares purchased at each separate price.</footnote>
        <footnote id="F2">Purchased 239 shares via ESPP on December 17, 2019</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Stacy L. Bogart, Vice President, General Counsel &amp; Secretary, Winnebago Industries, Inc. under Power of Attorney</signatureName>
        <signatureDate>2020-04-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
