The sea provides more than half the oxygen we breathe, food for billions of people and livelihoods for hundreds of millions. But giant corporations are plundering the world's oceans, aided by global finance and complicit states, following the neoliberal maxim of Blue Growth. The situation is dire: rampant exploitation and corruption now drive all aspects of the ocean economy, destroying communities, intensifying inequalities, and driving fish populations and other ocean life towards extinction. The Blue Commons is an urgent call for change, from a campaigning economist responsible for some of the most innovative solutions to inequality of recent times. From large nations bullying smaller nations into giving up eco-friendly fishing policies to the profiteering by the Crown Estate in commandeering much of the British seabed, the scale of the global problem is synthesised here for the first time, as well as a toolkit for all of us to rise up and tackle it. The oceans have been left out of calls for a Green New Deal but must be at the centre of the fight against climate change. How do we do it? By building a Blue Commons alternative: a transformative worldview and new set of proposals that prioritise the historic rights of local communities, the wellbeing of all people and, with it, the health of our oceans.