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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-02-24</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001096325</issuerCik>
        <issuerName>INTERSIL CORP/DE</issuerName>
        <issuerTradingSymbol>ISIL</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001338515</rptOwnerCik>
            <rptOwnerName>Sayiner Necip</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O INTERSIL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>1001 MURPHY RANCH ROAD</rptOwnerStreet2>
            <rptOwnerCity>MILPITAS</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>95035</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President and CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
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                <value>Common Stock</value>
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            <transactionDate>
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            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
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                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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                    <value>686481</value>
                </transactionShares>
                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
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            <transactionDate>
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            <transactionAmounts>
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            <expirationDate>
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            <underlyingSecurity>
                <underlyingSecurityTitle>
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                </underlyingSecurityShares>
            </underlyingSecurity>
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                </directOrIndirectOwnership>
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        </derivativeTransaction>
        <derivativeTransaction>
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                </transactionShares>
                <transactionPricePerShare>
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            <expirationDate>
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            <underlyingSecurity>
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                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
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                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
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            <transactionDate>
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            <deemedExecutionDate></deemedExecutionDate>
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                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>247204</value>
                </transactionShares>
                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <expirationDate>
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            <underlyingSecurity>
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                </underlyingSecurityTitle>
                <underlyingSecurityShares>
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                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
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            <ownershipNature>
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                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Deferred Stock Units (DSUs)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2014-04-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
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                    <value>26124</value>
                </transactionShares>
                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <exerciseDate>
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            <expirationDate>
                <footnoteId id="F5"/>
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            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>26124</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
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                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
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                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Deferred Stock Units (DSUs)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2015-04-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>61464</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F6"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F6"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>61464</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Deferred Stock Units (DSUs)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2016-04-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>89066</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F7"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F7"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>89066</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
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                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the Agreement and Plan of Merger, dated as of September 12, 2016, by and among Intersil Corporation (&quot;ISIL&quot;) and Renesas Electronics Corporation, as joined by Chapter One Company (as amended, the &quot;Merger Agreement&quot;), upon the closing of the merger on February 24, 2017 (the &quot;Closing&quot;), each outstanding share of common stock of ISIL was cancelled in exchange for the right to receive $22.50 in cash.</footnote>
        <footnote id="F2">Pursuant to the Merger Agreement, upon the Closing each unvested Performance-Based Market Stock Unit (&quot;MSU&quot;) was accelerated and the performance measurement period ended as of the Closing.  The number of shares acquired by reporting person is based on ISIL's Total Shareholder Return (&quot;TSR&quot;) performance relative to ISIL's peer group of companies over the measurement period.  The calculation for the number of shares earned by reporting person is based on 108,759 MSUs (at target) issued on 4/1/2014 multiplied by 150% payout.</footnote>
        <footnote id="F3">Pursuant to the Merger Agreement, upon the Closing each unvested Performance-Based Market Stock Unit (&quot;MSU&quot;) was accelerated and the performance measurement period ended as of the Closing.  The number of shares acquired by reporting person is based on ISIL's Total Shareholder Return (&quot;TSR&quot;) performance relative to ISIL's peer group of companies over the measurement period.  The calculation for the number of shares earned by reporting person is based on 127,944 MSUs (at target) issued on 4/1/2015 multiplied by 157.14% payout.</footnote>
        <footnote id="F4">Pursuant to the Merger Agreement, upon the Closing each unvested Performance-Based Market Stock Unit (&quot;MSU&quot;) was accelerated and the performance measurement period ended as of the Closing.  The number of shares acquired by reporting person is based on ISIL's Total Shareholder Return (&quot;TSR&quot;) performance relative to ISIL's peer group of companies over the measurement period.  The calculation for the number of shares earned by reporting person is based on 123,602 MSUs (at target) issued on 4/1/2016] multiplied by 200% payout.</footnote>
        <footnote id="F5">Pursuant to the Merger Agreement, upon the Closing each unvested DSU that was originally scheduled to vest in 2018 was converted into the right to receive a cash payment per share equal to $22.50 per DSU.  Unvested DSUs that were originally scheduled to vest in 2018, however, remain subject to the same vesting terms and conditions.</footnote>
        <footnote id="F6">Pursuant to the Merger Agreement, upon the Closing each unvested DSU that was originally scheduled to vest in 2019 (30,732 DSUs) was accelerated and cancelled in exchange for a cash payment per share equal to $22.50 per DSU. In addition, pursuant to the Merger Agreement, upon the Closing each unvested DSU that was originally scheduled to vest in 2018 (30,732 DSUs) was converted into the right to receive a cash payment per share equal to $22.50 per DSU.  Unvested DSUs that were originally scheduled to vest in 2018, however, remain subject to the same vesting terms and conditions.</footnote>
        <footnote id="F7">Pursuant to the Merger Agreement, upon the Closing each unvested DSU that was originally scheduled to vest in 2019 (29,688.66 DSUs) and 2020 (29,688.66 DSUs) was accelerated and cancelled in exchange for a cash payment per share equal to $22.50 per DSU. In addition, pursuant to the Merger Agreement, upon the Closing each unvested DSU that was originally scheduled to vest in 2018 (29,688.66) DSUs) was converted into the right to receive a cash payment per share equal to $22.50 per DSU.  Unvested DSUs that were originally scheduled to vest in 2018, however, remain subject to the same vesting terms and conditions.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Debbie Ceraolo-Johnson by Power of Attorney</signatureName>
        <signatureDate>2017-02-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
