Abstract Although the UN Charter does not explicitly provide for Peacekeeping Operations (PKOs), they have become one of the UN’s most important means of preserving peace and international security. Some of the greatest threats to international peace and security do not occur on ‘UN Member States territory’, but at sea. The internationally significant and long-standing phenomenon of maritime piracy initially led to international action off the coast of Somalia, but other regions affected by criminal acts at sea (including the Straits of Malacca, the Gulf of Guinea and the Mediterranean Sea) are reinforcing the need for international action. In most of these situations, the UN has not acted directly, but called on multilateral or regional bodies to do so. Earlier experiences in PKOs at sea, in Cambodia, East Timor, Haiti, and most recently in Lebanon, undertaken by the UN Interim Force in Lebanon (UNIFIL), begin to provide a basis for building a naval PKO doctrine. This article argues that it is legitimate for the UN to undertake direct action when facing threats to maritime security, making use of an empirical example and suggests that new instances of criminal acts at sea, such as those in the Gulf of Guinea, may best be dealt with under a direct UN mandate.