Despite adoption by Regional Fisheries Organisations of numerous measures aimed at deterring, preventing and eliminating illegal, unreported and unregulated fishing, IUU fishing by vessels flagged in both member and non-member states continues. None of the reciprocal boarding and inspection schemes adopted by RFOs provides for a positive right of arrest, detention or prosecution by a non-flag state in the event the flag states fails or refuses to take such action itself. The law of countermeasures may, however, provide legal justification for at-sea measures including boarding, inspection and possibly arrest and prosecution by members of RFOs against non-flag vessels of both member and non-member states. Although by nature an exceptional remedy, where the criteria discussed in this article are fulfilled, countermeasures in the form of at-sea enforcement may be an effective, and legal, tool in the fight against IUU fishing.