Global coordination towards marine conservation is likely to be more efficient than unilateral efforts, but no institution currently allows for nations to cooperate over the conservation of the marine environment. I use ecological principles, global empirical data on biodiversity and fisheries, and environmental economics to propose and analyze a new institution where nations can trade conservation obligations within reasonable ecological constraints: a global market for marine conservation. We describe the challenges and solutions to designing a market for marine conservation, and provide an example of how to build such a market and estimate the gains from trade.