Abstract: Why has Japan attempted to promote Asian security multilateralism for over two decades despite its open acknowledgment of the vital centrality of the US–Japan alliance in its overall foreign and security policy? Why has Japan’s pursuit of regional security multilateralism sometimes swayed between an inclusive ‘Asia-Pacific’ (with the US) and an exclusive ‘East Asian’ format (without the US)? This article examines the nature of Japan’s approach to Asian security multilateralism through a new analytical model based on the decentering/recentering framework and major theoretical assumptions deriving from neoclassical realism. It unpacks the process by which Japanese policy-makers have come to recognize Asian security multilateralism as a means of advancing perceived political and security interests, arising from international pressures and opportunities. The analysis is divided into three phases; (1) Japan’s leading role in the formation of the ASEAN Regional Forum (1991–1994), (2) Japan’s initiatives for the establishment of the ASEAN Plus Three and the East Asian Summit (1997–2005), and (3) Japan’s renewed focus on the EAS with US membership (2010-onwards). The article reveals the changing dynamics of the ideas and motivations behind Japan’s initiatives for Asian security multilateralism.