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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-01-31</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000025793</issuerCik>
        <issuerName>COSTA INC</issuerName>
        <issuerTradingSymbol>ATX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001245396</rptOwnerCik>
            <rptOwnerName>WHALEN DAVID G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O COSTA INC.</rptOwnerStreet1>
            <rptOwnerStreet2>24 ALBION ROAD, SUITE 330</rptOwnerStreet2>
            <rptOwnerCity>LINCOLN</rptOwnerCity>
            <rptOwnerState>RI</rptOwnerState>
            <rptOwnerZipCode>02865</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>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-01-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>533823</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.155</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2014-01-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>534351</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2015-01-08</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>534351</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Includes 50,000 shares of restricted stock that were previously granted to the Reporting Person.</footnote>
        <footnote id="F2">Pursuant to the terms of the Agreement and Plan of Merger (the &quot;Merger Agreement&quot;), dated as of November 7, 2013, by and among Costa Inc., a Rhode Island corporation (the &quot;Company&quot;), Essilor International SA, a French societe anonyme (&quot;Essilor&quot;), and GWH Acquisition Sub Inc., a Rhode Island corporation and an indirect wholly-owned subsidiary of Essilor (&quot;Merger Sub&quot;), at the effective time of the merger, each outstanding share of the Company's Class A Common Stock, par value $1.00 per share, was converted at into the right to receive $21.50 in cash (the &quot;Merger Consideration&quot;).  In addition, at the effective time of the merger, each outstanding share of restricted stock was cancelled and converted into the right to receive the Merger Consideration.</footnote>
        <footnote id="F3">This option was fully vested and exercisable prior to the effective time of the merger.</footnote>
        <footnote id="F4">Pursuant to the terms of the Merger Agreement, each outstanding option to purchase the Company's Class A Common Stock was cancelled and converted as of the effective time of the merger into the right to receive an amount in cash determined by multiplying (i) the excess, if any, of the Merger Consideration over the per-share exercise price of the Class A Common Stock subject to such stock option by (ii) the number of shares of Class A Common Stock subject to such option.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Tina Benik, as attorney in fact for David Whalen</signatureName>
        <signatureDate>2014-02-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
