Between the 1950s and 1960s, a comprehensive development plan existed concerning the Lower Mekong River Basin. Questions revolved around who devised what concept for the development of the Lower Mekong River, and how these concepts were implemented. In this article, we first analyzed the processes leading to Japan’s participation in the comprehensive development plan. Next, as part of the tributary development plan for this initiative, we analyzed the processes of the formation and subsequent development of the catchment area’s development plan for the Kingdom of Cambodia’s Prek Thnot River by multilateral development assistance as led by Japan. The development of the Prek Thnot River Power and Irrigation Project stopped as a result of the 1970 Cambodian Civil War and remains incomplete. Therefore, we analyzed the planning potential of the Prek Thnot River’s development plan from current viewpoints. What is made clear from the analyses is that both the basic philosophy and design philosophy behind the Mekong River Basin’s development initiative is relevant to today, and the development should not be conducted as a domestic matter, but should be conducted in continuation of its conceptual framework of international significance, as it concerns the suburb countries of the Mekong River.