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. Increasingly, RBOs are on the agenda of policy-makers who accord them a key role in promoting cooperation over shared water resources. Despite the increased attention paid to RBOs in international relations and water scholarship, there has been little focus on definitions and conceptualization of RBOs. This has challenged research around RBOs in both methodological and theoretical ways. This paper aims to bridge this gap by offering a theoretically-grounded definition of a River Basin Organization. We do so deductively, building from the larger institutionalist research and international water resources governance literature. Our definition identifies three broad categories of constitutive elements: internationalization, institutionalization and governance. We apply this definition to potential cases to better identify the extent of RBOs around the world today. We outline which cases qualify as RBOs and which cases fail to meet our constitutive criteria and why. We conclude by crafting an agenda for future research around RBOs that can considerably benefit from a more theoretically-grounded understanding of RBOs.