Although the Association of Southeast Asian Nations (ASEAN), China, and Japan commonly recognize that development of the Mekong region is indispensable for achieving the smooth promotion of regional integration in East Asia, their approaches to this issue have been diverse and redundant. While ASEAN has exhibited interest in Mekong development since the mid-1990s, its members’ commitments have showed significant disparity. The Chinese government has identified close links with the Mekong region as a key to advance political and economic linkages as well as to sustain the development of its underdeveloped southern areas. Japan’s Mekong policy has shifted from developmental to geopolitical, combining formal institutions, financial resources, and normative ideas. Such a strategic orientation aims to balance China’s growing influence by fostering direct political linkages with the Mekong countries. Weak coordination in approach to and interests in Mekong development has had negative impacts on institution building in East Asia. ASEAN’s limitation to coordinate development programs has undermined its credibility as the central body to advance institution building in the region. The different approaches of China and Japan have intensified rivalry on institution building in East Asia, disturbing the evolution of ASEAN+3 institutions in the development field.