The Commission may be joined by:

• Any government that is a member of the United Nations (UN)
• Any government that is a member of a Specialized Agency of the United Nations
• Any inter-governmental economic integration organization constituted by States that have transferred to it competence over the matters governed by the ICCAT Convention

To become a Contracting Party, an instrument of adherence to the International Convention for the Conservation of Atlantic Tunas must be deposited with the Director-General of the Food and Agriculture Organization of the United Nations (FAO). Membership becomes effective on the date that the instrument is deposited.

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The Commission can also grant the special status of Cooperator. Cooperators have many of the same rights and obligations that Contracting Parties have.

The procedures and criteria for attaining this status are laid out in the 2003 Recommendation by ICCAT on criteria for attaining the status of Cooperating non-Contracting Party, Entity or Fishing Entity in ICCAT. Those wishing to attain Cooperating Status should make their application through the Secretariat 90 days before the annual meeting, which usually takes place in November.