There are critical direct and indirect, positive and negative links between the ocean and human wellbeing at multiple scales. Many of the ecosystem services the ocean provides to humans, including fisheries, are common pool resources that are non-excludable but rival or subtractive. Others, including climate regulation, are non-rival and non-excludable pure public goods. Resources without any restriction or condition for their use, are known as open-access resources. Unrestricted access generally leads to unsustainable use. Many ocean ecosystems are open access, blue commons. They range in spatial scale from national, such as mangroves and coral reefs, to the open seas within Exclusive Economic Zones (EEZs), to Areas Beyond National Jurisdiction (ABNJ). There are also many species that migrate between national jurisdictions or between national and global blue commons. The sustainable stewardship of these blue commons is one of the most important current global governance challenges. We describe in detail how to create a Common Asset Trust (CAT) for blue commons and how such a system would operate. Finally, we propose CATs to manage blue commons at different spatial scales, including coastal reefs, EEZs, and ABNJ.