The Mekong Basin water management regime has existed since 1957, with three distinct periods. The Mekong Committee (1957-1975) pursued integrated basin development, which was not achieved due to geopolitical changes after the Vietnam War. During the Interim Mekong Committee era (1978-1992), the Mekong countries pursued independent water resources development, creating conflicts over water allocation. The Mekong River Commission (1995-present) aims to help member states share water in the post-Cold War era. The institutional framework of the Mekong River Commission reflects the historical legacy of the Mekong regime and must be understood in this context. Regional geopolitics, international development assistance, and water management norms are used to explain the evolution of the Mekong regime over four decades.