<p>Transboundary rivers, traversing multiple national borders, integrate sovereign states into a unified ecological system, complicating water resource governance amid rising global water scarcity and geopolitical tensions. Consequently, transboundary river governance exemplifies the public resource dilemma. This study, framed by constructivist international relations theory, examines the Lancang-Mekong River Basin as a case study, using data from multiple sources and socioeconomic indicators to explore the evolution of collective identity among riparian countries and its influencing factors. Key findings include: (1) The collective identity of riparian countries evolved in three phases: emergence (1971–1991), formation (1992–2014), and development (2015–2022). During this process, basin governance evolved from limited mechanisms to a more comprehensive, basin-wide system, with an expanded issue range and an increasing number of cooperation agreements. Cooperative attitudes transitioned from broadly positive to differentiated, ultimately aligning more favorably. (2) Economic interdependence is critical to the formation of collective identity among riparian countries, while diplomatic alignment enhances cooperation. (3) Extreme weather events and political globalization exert dual effects on collective identity formation: extreme weather fosters cooperation but also prioritizes domestic recovery, complicating agreements and expanding issues. Political globalization has facilitated institutionalization and normalization of cooperation, though external involvement has deepened divisions in cooperative attitudes. This study contributes to theoretical perspectives on transboundary river governance and supports collective action in global environmental governance.</p>