Despite global policy commitments to preserve Earth's marine biodiversity, many species are in a state of decline. Using data on 22,885 marine species, the authors identify 8.5 million km² of priority areas that complement existing areas of conservation and biodiversity importance. New conservation priorities are found in over half (56%) of all coastal nations, including key priority regions in the northwest Pacific Ocean and Atlantic Ocean. The authors identify where different conservation actions, ranging from marine protected areas to broader policy approaches, might best overcome anthropogenic threats to these areas. This analysis shows 26%–41% of the ocean (depending on targets used for species representation) needs to be effectively managed and conserved through a combination of site-based actions and broad policy responses to achieve global conservation and sustainable development agendas.