The Mar Menor, Europe's largest saltwater coastal lagoon, was long sustained by high salinity and low-nutrient waters that supported remarkable biodiversity. Since the late twentieth century, however, intensive tourism, industrial agriculture linked to the Tagus–Segura water transfer, and legacy mining pollution have driven accelerating ecological degradation. The eutrophication crisis of 2016 and the mass anoxic events of 2019 and 2021, which caused extensive marine die-offs, marked a profound ecological and political rupture. In response, a civic movement led by Teresa Vicente achieved an unprecedented outcome in 2022: the lagoon was granted legal personality, becoming the first ecosystem in Europe to obtain such status. This Element examines the social, legal, and scientific transformations surrounding this case and argues that recognising the lagoon as a subject opens new possibilities for rethinking human–nature relations and imagining more-than-human political communities grounded in ecological justice.