<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>5</documentType>
<periodOfReport>2003-09-30</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <form3HoldingsReported>0</form3HoldingsReported>

    <form4TransactionsReported>1</form4TransactionsReported>

    <issuer>
        <issuerCik>0000353567</issuerCik>
        <issuerName>ICO INC</issuerName>
        <issuerTradingSymbol>ICOC</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001218559</rptOwnerCik>
            <rptOwnerName>WILLOUGHBY WILLIAM C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>6250 COVERED BRIDGE RD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>PIPERSVILLE</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>18947</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector>
<isOfficer>0</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle></officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable>
<nonDerivativeHolding>
<securityTitle>
<value>Common Stock</value>
</securityTitle>
<transactionCoding>
<transactionFormType></transactionFormType>
</transactionCoding>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>794867</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeHolding>
<nonDerivativeHolding>
<securityTitle>
<value>IRA - Common Stock</value>
</securityTitle>
<transactionCoding>
<transactionFormType></transactionFormType>
</transactionCoding>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>4230</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeHolding>
<nonDerivativeHolding>
<securityTitle>
<value>Common Stock</value>
</securityTitle>
<transactionCoding>
<transactionFormType></transactionFormType>
</transactionCoding>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>71495</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>by Spouse</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeHolding>
<nonDerivativeHolding>
<securityTitle>
<value>Common Stock</value>
</securityTitle>
<transactionCoding>
<transactionFormType></transactionFormType>
</transactionCoding>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>21212</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>Custodian for William B. Willoughby</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeHolding>
<nonDerivativeHolding>
<securityTitle>
<value>Common Stock</value>
</securityTitle>
<transactionCoding>
<transactionFormType></transactionFormType>
</transactionCoding>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>4898</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>Spouse custodian for William B. Willoughby</value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeHolding>
<nonDerivativeHolding>
<securityTitle>
<value>Common Stock</value>
</securityTitle>
<transactionCoding>
<transactionFormType></transactionFormType>
</transactionCoding>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>1537862</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value>Trusts established for the beneift of Mr. Willoughby and his sister Catherine W. Stephens</value>
<footnoteId id="F1"/>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeHolding>
</nonDerivativeTable>
<derivativeTable>
<derivativeHolding>
<securityTitle>
<value>Non-employee Stock Option Plan</value>
</securityTitle>
<conversionOrExercisePrice>
<value>2.70</value>
</conversionOrExercisePrice>
<transactionCoding>
<transactionFormType></transactionFormType>
</transactionCoding>
<exerciseDate>
<value>2001-06-20</value>
</exerciseDate>
<expirationDate>
<value>2011-06-20</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>5000</value>
</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>5000</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</derivativeHolding>
<derivativeHolding>
<securityTitle>
<value>Non-employee Stock Option Plan</value>
</securityTitle>
<conversionOrExercisePrice>
<value>1.35</value>
</conversionOrExercisePrice>
<transactionCoding>
<transactionFormType></transactionFormType>
</transactionCoding>
<exerciseDate>
<value>2002-03-17</value>
</exerciseDate>
<expirationDate>
<value>2012-03-17</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>5000</value>
</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>5000</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</derivativeHolding>
<derivativeTransaction>
<securityTitle>
<value>Non-Employee Stock Option Plan</value>
</securityTitle>
<conversionOrExercisePrice>
<value>1.195</value>
</conversionOrExercisePrice>
<transactionDate>
<value>2003-03-03</value>
</transactionDate>
<deemedExecutionDate>
</deemedExecutionDate>
<transactionCoding>
<transactionFormType>4</transactionFormType>
<transactionCode>A</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionTimeliness>
<value>L</value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>5000</value>
</transactionShares>
<transactionPricePerShare>
<value>1.195</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode>
<value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<exerciseDate>
<value>2003-03-03</value>
</exerciseDate>
<expirationDate>
<value>2013-03-03</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>5000</value>
</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>5000</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</derivativeTransaction>
</derivativeTable>
<footnotes>
<footnote id="F1">An aggregate of 768,931 shares of Common stock is held in two of these trusts for the benefit of Mr. Willoughby and an aggregate of 768,931 shares of Common Stock is held in two of these trusts for the benefit of Ms. Stephens. Mr. Willoughby and Ms. Stephens are each a cotrustee of these trusts.</footnote>
</footnotes>
<remarks></remarks>
<ownerSignature>
<signatureName>William C. Willoughby</signatureName>
<signatureDate>2004-01-22</signatureDate>
</ownerSignature>
</ownershipDocument>

