This paper introduces the concept of commoning in circular economies, and explores how commons reproduce over time. The starting point is that commoning can have an important role in fostering circular economies and sustainable and socially-inclusive development. By commoning, we refer to local stakeholders working collectively to preserve or restore their natural resource base to generate benefits that are locally shared. Through the analysis of a specific case of a group of commoners’ associations in Galicia (Spain), the paper describes and discusses the development, and ultimate unravelling, of an innovative and decentralized waste management project to convert waste biomass from the monte (often-neglected upland green spaces, largely consisting of brush and trees) into compost. In order to make this composting project economically viable the possibility of collecting and processing urban green waste was also explored. While the project’s application of the principles of a circular economy had the potential to bring locally-shared economic and ecological benefits, and foster territorial prosperity and resilience, it was ultimately frustrated by questions of scale, administrative and regulatory barriers, competing and conflicting land-use claims and financial cutbacks in the public sector.