From enslaved people capturing vessels during the Middle Passage to Greenpeace’s and Sea Shepherd’s famous anti-whaling campaigns in the latter half of the twentieth century – resistance at sea is a phenomenon that is as old as human seafaring. Yet, while social movement scholars have a long-standing interest in transnational movements and what it means to act politically beyond the borders of nation-states, much less is known about social movements acting outside of national territory, in the international waters of the planetary ocean. This article addresses this gap by bringing the phenomenon of sea-oriented civil society to the attention of social movement scholars and proposing a conceptual framework for understanding ocean activism. Bringing together currently scattered accounts of maritime movements with relevant literature from the recent ocean turn across the social and political sciences, the article argues that we may understand ocean activism along three defining, sea-specific characteristics: (1) material specificity (due to the sea’s elemental qualities), (2) technological dependency (from ships to AIS), and (3) extra-territoriality (outside national jurisdiction). The paper contributes to discussions of transnational social movements in particular, demonstrating that scholarly attention to the ocean enables new perspectives on their spatial politics, media and data practices, and materiality. Thus, addressing social movement scholarship’s terrestrial bias does not only allow us to begin to conceptualize ocean activism but opens up paths to better understand political acts in extra-national terrains, in and beyond the sea.