Cumulative human impacts have led to the degradation of marine ecosystems and the decline of biodiversity in the European and contiguous seas. Effective conservation measures are urgently needed to reverse these trends. Conservation must entail societal choices, underpinned by human values and worldviews that differ between the countries bordering these seas. Social, economic and political heterogeneity adds to the challenge of balancing conservation with sustainable use of the seas. Comprehensive macro-regional coordination is needed to ensure effective conservation of marine ecosystems and biodiversity of this region. Under the European Union Horizon 2020 framework programme, the MarCons COST action aims to promote collaborative research to support marine management, conservation planning and policy development. This will be achieved by developing novel methods and tools to close knowledge gaps and advance marine conservation science. This action will provide support for the development of macro-regional and national policies through six key actions: to develop tools to analyse cumulative human impacts; to identify critical scientific and technical gaps in conservation efforts; to improve the resilience of the marine environment to global change and biological invasions; to develop frameworks for integrated conservation planning across terrestrial, freshwater, and marine environments; to coordinate marine conservation policy across national boundaries; and to identify effective governance approaches for marine protected area management. Achieving the objectives of these actions will facilitate the integration of marine conservation policy into macro-regional maritime spatial planning agendas for the European and contiguous seas, thereby offsetting the loss of biodiversity and ecosystem services in this region.