Over the past decade, Japan has reformed its foreign and security posture at a pace not witnessed since the first decades of the postwar period. The second Abe Shinzo government (2012–2020) established new national security institutions and laws, reinterpreted the legal foundation for its alliances, revised the development aid charter, removed bans on weapon exports, and aligned explicit security, economic and development policies in broad initiatives across the Indo-Pacific region. Taken together, these moves represent a significant expansion of the boundaries of Japanese foreign policy. How has this been possible? Situated in the field of international relations (IR), with a focus on identity and how repertoires can contribute to our understanding of global interactions, this research provides a rich empirical investigation into the expansion of Japanese foreign policy by revealing the intricacies of the language and practice of policy legitimation. Theoretically, this research draws inspiration from the literature on repertoires. While the repertoire concept stems from the sociological study of social movements, applied to an IR context it allows us to study the reflexive use of and tinkering with the instruments available for the conduct of foreign policy. Actors are constrained by the instruments (economic, military, diplomatic, cultural) available to them, but can also develop and recombine them to effect change over time. To better grasp changes in Japanese foreign policymaking over the past decade, this thesis employs discourse and practice analysis to investigate the tension between national identity, the decision-making processes behind the inclusion of certain instruments in Japan’s foreign policy, the legitimation of policy changes, and the actual, daily execution of foreign policy instruments.  To address the question of how instruments in policy repertoires are legitimated, promoted and enacted, the thesis analyzes three aspects of contemporary Japanese foreign policy—public contests, public promotions and on-the-ground practices—and considers specific foreign policy instruments (long-range cruise missiles and official development assistance) and various sites of practice (the Japanese parliament, development cooperation festivals and Myanmar). The thesis comprises an introductory chapter and four related articles with independent research inquiries. As a whole, it demonstrates that delving into the detail of foreign policy legitimation through the study of repertoires can bring to the fore new knowledge about the conduct of foreign policy and advance our understanding of the foreign policy expansions of one of the most prominent states in a region of central importance to international politics. Specifically, it finds that dominant narratives about Japan as a foreign policy actor provide a necessary foundation for unorthodox policy proposals. The analysis finds both path dependency and innovation in Japan’s foreign policy repertoire. Its findings lend evidence to scholarship on foreign policy, legitimation and identity, as well as to the debate on the expansion of Japanese foreign policy, and are of relevance to both scholars and foreign policy practitioners.