This article argues that Japan matters crucially in the evolving East Asian security order because it is embedded both in the structural transition and the ongoing regional strategies to manage it. The post-Cold War East Asian order transition centres on the disintegration of the post-Second World War Great Power bargain that saw Japan subjecting itself to extraordinary strategic constraint under the US alliance, leaving the conundrum of how to negotiate a new bargain that would keep the peace between Japan and China. To manage the uncertainties of this transition, East Asian states have adopted a three-pronged strategy of: maintaining US military preponderance; socializing China as a responsible regional great power; and cultivating regionalism as the basis for a long-term East Asian security community. Japan provides essential public goods for each of these three elements: it keeps the US anchored in East Asia with its security treaty; it is the one major regional power that can and has helped to constrain the potential excesses of growing Chinese power while at the same time crucially engaging with and helping to socialize China; and its economic and political participation is critical for meaningful regionalism and regional integration. It does not need to be a fully fledged, ‘normal’ Great Power in order to carry out these roles. As the region tries to mediate the growing security dilemma among the three great powers, Japan's importance to regional security will only grow.