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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-11-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000913665</issuerCik>
        <issuerName>TERRESTAR CORP</issuerName>
        <issuerTradingSymbol>TSTRQ.PK</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001233563</rptOwnerCik>
            <rptOwnerName>HARBINGER CAPITAL PARTNERS MASTER FUND I, LTD.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O INTERNATIONAL FUND SVS. (IRELAND) LT</rptOwnerStreet1>
            <rptOwnerStreet2>78 SIR JOHN ROGERSON'S QUAY</rptOwnerStreet2>
            <rptOwnerCity>DUBLIN 2</rptOwnerCity>
            <rptOwnerState>L2</rptOwnerState>
            <rptOwnerZipCode>00000</rptOwnerZipCode>
            <rptOwnerStateDescription>IRELAND</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001371517</rptOwnerCik>
            <rptOwnerName>HARBINGER CAPITAL PARTNERS SPECIAL SITUATIONS FUND, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>450 PARK AVENUE, 30TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Form filed by More than One Re</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001371518</rptOwnerCik>
            <rptOwnerName>HARBINGER CAPITAL PARTNERS SPECIAL SITUATIONS GP, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>450 PARK AVENUE, 30TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Director, Chairman and CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001456927</rptOwnerCik>
            <rptOwnerName>HARBINGER HOLDINGS, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>450 PARK AVENUE, 30TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001233569</rptOwnerCik>
            <rptOwnerName>FALCONE PHILIP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>450 PARK AVENUE, 30TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001457298</rptOwnerCik>
            <rptOwnerName>HARBINGER CAPITAL PARTNERS LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>450 PARK AVENUE, 30TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-11-23</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>274251</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.1936</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>36774923</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F5"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-11-23</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>70749</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.1936</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10257108</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">These shares of the Issuer's common stock, par value $0.01 per share (the &quot;Shares&quot;), are owned by Harbinger Capital Partners Master Fund I, Ltd. (the &quot;Master Fund&quot;), which is a Reporting Person.</footnote>
        <footnote id="F2">These Shares may be deemed to be indirectly beneficially owned by the following, each of whom is a Reporting Person:  Harbinger Capital Partners LLC (&quot;Harbinger  LLC&quot;), the investment manager of the Master Fund; Harbinger Holdings, LLC (&quot;Harbinger Holdings&quot;), the manager of Harbinger LLC; and Philip Falcone (&quot;Mr. Falcone&quot;), the managing member of Harbinger Holdings and the portfolio manager of the Master Fund.</footnote>
        <footnote id="F3">These Shares are owned by Harbinger Capital Partners Special Situations Fund, L.P. (the &quot;Special Fund&quot;), which is a Reporting Person.</footnote>
        <footnote id="F4">These Shares may be deemed to be indirectly beneficially owned by the following, each of whom is a Reporting Person:  Harbinger Capital Partners Special Situations GP, LLC (&quot;HCPSS&quot;), the general partner of the Special Fund; Harbinger Holdings, the managing member of HCPSS; and Mr. Falcone, the managing member of Harbinger Holdings and the portfolio manager of the Special Fund.</footnote>
        <footnote id="F5">Each Reporting Person disclaims beneficial ownership of the reported securities except to the extent of his or its pecuniary interest therein, and this report shall not be deemed an admission that such Reporting Person is the beneficial owner of the securities for purposes of Section 16 of the Securities Exchange Act of 1934, or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>HARBINGER CAPITAL PARTNERS MASTER FUND I, LTD., By: HARBINGER CAPITAL PARTNERS LLC, By: HARBINGER HOLDINGS, LLC, Manager, By: /s/ Philip Falcone, Managing Member</signatureName>
        <signatureDate>2010-11-26</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>HARBINGER CAPITAL PARTNERS LLC, By: HARBINGER HOLDINGS, LLC, Manager, By: /s/ Philip Falcone, Managing Member</signatureName>
        <signatureDate>2010-11-26</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>HARBINGER CAPITAL PARTNERS SPECIAL SITUATIONS FUND, L.P., By: HARBINGER CAPITAL PARTNERS SPECIAL SITUATIONS GP, LLC, By: HARBINGER HOLDINGS, LLC, Managing Member, By: /s/ Philip Falcone, Managing Member</signatureName>
        <signatureDate>2010-11-26</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>HARBINGER CAPITAL PARTNERS SPECIAL SITUATIONS GP, LLC, By: HARBINGER HOLDINGS, LLC, Managing Member, By: /s/ Philip Falcone, Managing Member</signatureName>
        <signatureDate>2010-11-26</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>HARBINGER HOLDINGS, LLC, By: /s/ Philip Falcone, Managing Member</signatureName>
        <signatureDate>2010-11-26</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Philip Falcone</signatureName>
        <signatureDate>2010-11-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
