State security is a key concern for countries when dealing with transboundary river governance issues. While there is a considerable amount of literature on transboundary river cooperation in general, there has been little on transboundary cooperation and the damming of shared rivers in terms of their impacts on state security. As a result, this article seeks to answer the following questions in relation to the lower Mekong basin: (i) How is the damming process embedded in transboundary cooperation? (ii) How does this type of cooperation contribute to peace and security in the study area – the 3S river basin? And, (iii) how can transboundary cooperation be improved so as to address state security issues? To answer these questions, this study reviews the relevant literature and uses two case studies, one of the Yali Dam in Vietnam and the second of the Lower Sesan 2 Dam in Cambodia. In doing so, the article examines dam developments in the lower Mekong basin, their environmental and human security impacts, and how they contribute to or threaten state security in Cambodia. It concludes that dams on international rivers induce insecurity among states downstream and that cooperation between riparian states is key to addressing this insecurity.