In the greater metropolitan region of São Paulo, Brazil, close to 20 million people share water resources across multiple uses, from industry to recreation to basic household use. These resources have come under increasing strain due to urban expansion, poor planning, and, more recently, climate change. Conflicts among uses and their underlying principles are increasingly adjudicated in courts, with the Ministério Público (Public Prosecutor's Office) acting as a key advocate for both human rights and environmental protection. As legal interventions become more common in policy questions, how are justice principles shaping emerging approaches to water governance? And how are these types of cases provoking a reconsideration of the role of law in public administration? This paper analyzes the justice ramifications of an increasing use of legal discourses and practices to adjudicate water-related conflicts in São Paulo. It also, and conversely, shows how legal battles over water and sanitation force a shift in the behavior and perspectives of legal actors, and subsequently broaden the meaning of ‘justice.’