Current emission reduction pledges under the 2015 Paris Agreement are insufficient to keep global temperature “well below +2°C” in 2100 relative to pre-industrial levels and to reach targets of the United Nations Sustainable Development Goals. Increased political ambition is therefore required, as well as enhanced efforts in terms of both mitigation and ecosystem and human adaptation. There is growing evidence high- lighting both the role the ocean plays in mitigating anthropogenic climate change (i.e., absorption of atmospheric heat and anthropogenic carbon), and the cascading consequences on its chemistry and physics (i.e., ocean warming, acidification, deoxygenation, sea-level rise), ecosystems and ecosystem services. In such a context, a critical question arises: what are the ocean-based opportunities for climate action? In other words, what is the potential of the ocean and its ecosystems to reduce the causes of climate change and its impacts? This document summarises the main findings of The Ocean Solutions Initiative that assessed the potential of 13 ocean-based measures.