This chapter analyzes Japan’s experience with, and motivations for participating in, security multilateralism. It considers the historical legacies of a lack of multilateral interaction when international relations in East Asia were governed by the Sinocentric tributary system, the security multilateralism of the Washington system of the 1920s, Japan’s failed multilateral Greater East Asia Co-Prosperity Sphere during the Pacific War, as well as Tokyo’s security isolationism during the Cold War, before turning to Japan’s pivot from 1991 toward embracing regional security multilateralism. It argues that Japan’s promotion of security multilateralism since 1991 is part of a broader shift away from security isolationism and toward global and regional security engagement on bilateral and multilateral levels. Japan’s 1992 decision to begin participating in UN peacekeeping and its promotion of the ASEAN Regional Forum’s creation through the 1991 Nakayama proposal are examples of Japan’s post–Cold War security multilateralism. The chapter argues that Japan’s embrace of security multilateralism after the Cold War, like its embrace of security isolationism during the Cold War, has been driven by its reassurance strategy of convincing other East Asian nations that Japan can be trusted as a military power that will not repeat its pre-1945 expansionism. Moderating its alliance security dilemma vis-à-vis the US is another motivation for Japan’s promotion of security multilateralism. Since 2000 Japan has promoted the creation of other regional security multilateral forums, including the counter-piracy ReCAAP organization, the East Asian Summit, the ASEAN Defense Ministers’ Meeting Plus Dialogue Partners, and the Expanded ASEAN Maritime Forum.