Japan’s Environment Diplomacy in Southeast Asia within Free and Open Indo-Pacific

Background and objective: Amid intensifying U.S.-China strategic competition and the expansion of China’s Belt and Road Initiative (BRI), Southeast Asia has emerged as a pivotal arena in the evolving Indo-Pacific order. Within the framework of Japan’s Free and Open Indo-Pacific (FOIP) vision, environmental diplomacy has gained prominence as a strategic instrument to enhance Japan’s regional role beyond traditional economic and security domains. This article examines how Japan employs environmental diplomacy in Southeast Asia to advance its strategic objectives under FOIP.Methods: The study adopts a qualitative analytical approach grounded in neorealist balance-of-power theory and middle power diplomacy. It analyzes official policy documents, diplomatic initiatives, and key environmental cooperation projects between Japan and Southeast Asian countries to assess the strategic functions of environmental diplomacy.Results: The findings suggest that environmental diplomacy enables Japan to project influence through norm diffusion, high-quality infrastructure standards, and cooperative solutions to shared environmental challenges such as climate change and marine pollution. By operating in a less politically sensitive domain, Japan mitigates strategic risks while expanding its presence and reinforcing its image as a responsible and non-aggressive regional leader.Conclusion: Environmental diplomacy under FOIP functions as a form of soft balancing, allowing Japan to subtly counterbalance China’s growing influence without undermining ASEAN centrality or provoking overt geopolitical confrontation. This strategy underscores the evolving role of environmental cooperation as a critical component of Japan’s middle power diplomacy in the Indo-Pacific.