Security cooperation between Japan and Australia has evolved since 2007. With the steady development of their alliances with the United States, their bilateral ‘Special Strategic Partnership’ now has become the most advanced ‘spoke and spoke’ relationship in the region. This chapter examines the impact of a series of reforms to Japan’s domestic security policy on its bilateral cooperation with Australia, and reveals the impact of changes in the regional security environment on these domestic reforms. It also demonstrates how changes within the Indo-Pacific security environment have helped motivate, and enable, greater Japan–Australia security cooperation. The chapter also highlights some major challenges for the Japanese government in pursuing closer security engagement and cooperation with Australia. Aside from the possible, but unlikely, formation of a formal security alliance between Japan and Australia, bilateral security cooperation nevertheless has grown in the midst of China’s ongoing challenge to the existing rules-based regional order, and the greater uncertainty over US intentions introduced by the Trump administration.