Abstract: Institutions that have been set up by riparian states to internationally govern shared water resources—international River Basin Organizations (RBOs)—play a key role in river basin governance. Despite an increased attention paid to RBOs in international relations and water scholarship, there has been little focus on defining and conceptualizing RBOs and, subsequently, on comprehensively identifying the RBOs that exist around the world. This has challenged research around RBOs in both methodological and theoretical ways. This paper aims to meet this challenge by offering a theoretically grounded definition of an international RBO and crafting a comprehensive list of international RBOs. We do so deductively, building from the larger neo-institutionalist research and international water resources governance literature. Our definition identifies three broad categories of constitutive elements: internationalization, institutionalization and governance.