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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-09-30</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001241199</issuerCik>
        <issuerName>CAPITALSOURCE INC</issuerName>
        <issuerTradingSymbol>CSE</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001257511</rptOwnerCik>
            <rptOwnerName>FREMDER ANDREW B</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O FARALLON CAPITAL MANAGEMENT, L.L.C.</rptOwnerStreet1>
            <rptOwnerStreet2>ONE MARITIME PLAZA, SUITE 2100</rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94111</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Member of Group Owning 10%</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-09-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>325</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>325</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>16140</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Each share of phantom stock represents the right to receive one share of common stock.</footnote>
        <footnote id="F2">The shares of common stock underlying these shares of phantom stock are payable upon termination of the reporting person's service as a director, a date selected by the reporting person or as provided in the Issuer's Amended and Restated Deferred Compensation Plan, a form of which was filed with the Securities and Exchange Commission on August 8, 2007, as Exhibit 10.21 to the Issuer's Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2007.</footnote>
        <footnote id="F3">The number of derivative securities reported in Column 9 includes 63 phantom stock units received by Fremder on or about September 30, 2008, pursuant to the Issuer's Amended and Restated Deferred Compensation Plan, of which 42 shares of phantom stock are vested and 21 shares of phantom stock vest on the date of the Issuer's 2009 annual meeting. On or about September 30, 2008, the Issuer paid a quarterly cash dividend to all of its holders of record of shares of common stock on September 17, 2008. The 63 phantom stock units represent the re-investment of the cash dividend payable to Fremder with respect to his existing phantom stock units pursuant to the Issuer's Amended and Restated Deferred Compensation Plan. The receipt of such additional phantom stock units was exempt from Section 16 of the Securities Exchange Act of 1934, as amended.</footnote>
        <footnote id="F4">As reported in the Form 4s filed by Fremder and certain entities and individuals affiliated with Farallon Capital Management, L.L.C. and Farallon Partners, L.L.C. on or about July 1, 2008, with respect to the Issuer's common stock (together, the &quot;Farallon Form 4s&quot;), if Fremder and the entities and individuals identified in the Farallon Form 4s were deemed members of a group holding equity securities of the Issuer, Fremder may be deemed to be a beneficial owner of the securities reported in the Farallon Form 4s. Fremder expressly disclaims that he is a member of such group and disclaims any beneficial ownership of any of the Issuer's securities reported in the Farallon Form 4s for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or otherwise.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Mark C. Wehrly, Attorney-in-Fact</signatureName>
        <signatureDate>2008-10-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
