This article uses two case studies to illustrate the subjection of indigenous peoples’ marine territories to a ‘double jeopardy’ of exclusion — jurisdictional and proprietary — through the legal and administrative practices of European ‘settler’ states in Australia and Canada. While the fiction of terra nullius as a legal rationale for refuting indigenous rights of property and governance has steadily eroded in recent decades, its counterpart mare nullius has proven, so far, more resistant. The authors examine how state conceptions of jurisdiction, property and boundary-making in coastal areas accomplish the distortion and fragmentation of the coastal and marine spaces of Torres Strait Islanders in northern Queensland, Australia, and of the Cree and Inuit peoples of James and Hudson Bays in northern Que´bec, Canada. Assumptions of land–sea continuity underlie these peoples’ cultural constructions of coastal and marine environments. In examining the progress that each has made in reasserting ownership and control of coast and sea, it seems that recognition and reinforcement of their institutions for managing marine spaces and resources offer the best prospect for reconnecting fractured jurisdictional domains, and for bringing about social equity, environmental protection, and self-determined regional development.