Scholars and practitioners now acknowledge that “MAD Water” systems (modular, adaptive, decentralized engineered infrastructures) will expand to meet human water needs under future climate change, migration, and urbanization scenarios. Yet social science research on existing MAD water infrastructures documents how the use and deployment of such systems often undermines water justice. Here we posit that identifying and analyzing moral economies for water provides one approach for scholars to understand—and possibly predict—when and why justice norms in MAD water systems break down or become unstable. Moral economies are institutional arrangements in which people’s shared ideas of justice normatively shape how they distribute and exchange basic resources. We review the concept of moral economies, explain an operational framework for researching moral economies, and illustrate how moral economies function within three already-operating MAD water systems today: water sharing arrangements, informal water vending markets, and small-scale water commons. We argue that when moral economies are embedded and operating successfully within MAD water systems, they can create check-and-balance mechanisms against injustice. But when moral economies are absent or failing in MAD water systems, water injustices often prevail. As such, the moral economies framework provides not only a tool for analysis, but also a possible language and pathway forward toward organizing for justice in MAD water systems.