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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2015-06-17</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001447599</issuerCik>
        <issuerName>FITBIT INC</issuerName>
        <issuerTradingSymbol>FIT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001645372</rptOwnerCik>
            <rptOwnerName>MURRAY STEVEN JOSEPH</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SOFTBANK CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>38 GLEN AVENUE</rptOwnerStreet2>
            <rptOwnerCity>NEWTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02459</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Series D Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10168572</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By SoftBank PrinceVille Investments, LP</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The Convertible Preferred Stock shall automatically convert into Class B Common Stock on a one for one basis immediately prior to the completion of the Issuer's initial public offering and has no expiration date.</footnote>
        <footnote id="F2">Each share of Class B Common Stock is convertible at any time at the option of the Reporting Person into one share of Class A Common Stock and has no expiration date. Upon the closing of the issuer's sale of its Class A Common Stock in its firm commitment underwritten initial public offering pursuant to a registration statement on Form S-1 (File No. 333-203941) under the Securities Act of 1933, as amended, the Class B Common Stock will convert automatically into Class A Common Stock on the same basis upon the earlier of: (i) any transfer, whether or not for value, except for certain &quot;Permitted Transfers&quot; as defined in the Issuer's restated certificate of incorporation in effect as of the date hereof, (ii) the affirmative vote of the holders of Class B Common Stock representing not less than a majority of the outstanding shares of Class B Common Stock, or (iii) June 17, 2027.</footnote>
        <footnote id="F3">SB PV GP L.P. is the general partner of SoftBank PrinceVille Investments, L.P. and SB PV GP LLC is the general partner of SB PV GP L.P. The managing members of SB PV GP LLC are Ronald D. Fisher, Kabir Misra, and Steven Murray, a member of the Issuer's board of directors, and, therefore, may be deemed to share voting and dispositive power over the shares held by SoftBank PrinceVille Investments, L.P.  Messrs. Fisher, Misra, and Murray disclaim beneficial ownership of these securities except to the extent of their pecuniary interests therein, and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of all of the reported shares for purposes of Section 16 or for any other purpose.</footnote>
    </footnotes>

    <remarks>Exhibit List - Exhibit 24 - Power of Attorney</remarks>

    <ownerSignature>
        <signatureName>/s/ Juliana Chen, attorney-in-fact</signatureName>
        <signatureDate>2015-06-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
