This paper offers an examination of the complex dynamics within the Mekong River Basin, a region historically intertwined with the Mekong River’s abundance but now marked by a shift towards hydroelectric power as a resource. Focusing on the reluctance of China—a key player in the basin—to join the Mekong Agreement, this paper uncovers the environmental and social consequences of this non-cooperation. It discusses the historical context of international environmental law related to transboundary water resources, emphasizing how China’s actions challenge established principles and responsible resource management norms. Highlighting the significance of transboundary collaboration underscores the essential role it plays in ensuring the sustainable governance of shared water resources in the Mekong River Basin.