Marine governance is challenged by complex situations at regional seas and coastal areas of European countries where multiple stakeholder interests and numerous management options have to be balanced. In these situations an improved understanding of marine governance is crucial for ensuring sustainable development at sea. The main aim of this synthesis article is to increase present understanding of processes of regionalization in EU marine governance based on the contributions in this special issue. Regionalization is defined as the integration and cooperation of maritime activities, policies and actors at the level of the regional seas. The contributions in this special issue analyse processes of regionalization within different maritime sectors from a diversity of social scientific disciplines to unravel different forms and types of regionalization in marine governance. The main finding is that there is still a relatively large mismatch between the vision on ecosystem-based management expressed in EU marine policies and the implementation of EBM within sectoral maritime activities. A reduction of such a gap between EU marine policies and sectoral management fully depends on efforts at regional level to coordinate and to integrate the different sector policy processes.