This article explores the legal implications of intelligence collection operations at sea. It concludes that in terms of the international law of the sea, intelligence collection that occurs outside of the territorial sea is lawful. Furthermore, even intelligence collection by a foreign ship inside the territorial sea, while a violation of State sovereignty, may not violate the law of the sea, per se. Additionally, within the territorial sea, coastal States are limited in the measures they may take against foreign-flagged submarines and surface warships collecting intelligence since those activities do not constitute an armed attack or even the use of force in international law and the platforms are protected by sovereign immunity. Ultimately, what these findings suggest is that the international law of the sea does not prohibit intelligence collection in the maritime domain, and it has limited utility in addressing larger questions about the lawfulness of intelligence collection activities.