This dissertation is a case-study-led analysis of the role of special envoys in Japanese postwar diplomacy based on multiarchival research in Japan, the UK, the US, and South Korea. It examines how and why, in the early postwar period, Japanese prime ministers often relied on trusted individuals with no diplomatic experience or official affiliation as their personal ‘special envoys’ (technically, ‘executive agents’) – an informal, extra-institutional tool of diplomacy operating outside of established bureaucratic channels. By exploring the promises and pitfalls of ‘one man diplomacy’, it advances an innovative interpretation of envoy diplomacy as both a ‘post-imperial’ and a ‘presidential’ phenomenon: a holdover of imperial-era ways of conducting foreign policy as well as a stretch of prime ministerial power, channelling great power ambitions and aspirations of executive leadership over policy-making. Special envoys played a central (yet understudied) role in some of the most critical junctures of postwar Japanese history. This dissertation examines their role in the mediation of the Indonesia-Malaysia dispute of 1963-66, led by LDP vice-president Kawashima Shōjirō; the 1972 reversion of Okinawa from US military administration, negotiated by scholar Wakaizumi Kei; and the 1983 rapprochement with South Korea, liaised by business strategist Sejima Ryūzō. Special envoys – be they businessmen, lawmakers or scholars – challenged the two fundamental tenets of Japan’s postwar foreign policy: diplomatic restraint and bureaucratic control over policy-making. Being unconstrained by institutional accountability, formal process, and often even public scrutiny, they pursued an assertive diplomatic style and pushed the limits of prime ministerial authority in foreign policy. Envoy diplomacy offers a rare glimpse into the political fault lines that accompanied the self-definition of the postwar Japanese state, revealing the contested cleavages in defining Japan’s role in the world and the prerogatives of its political leadership in foreign policy. As late Premier Abe Shinzō’s ambitious vision for Japan’s international role (as a ‘tier one’ nation) and proactive leadership style (as a ‘diplomat in chief’) have shown, these questions remain at the forefront of Japan’s foreign policy debate to this day. It is by looking at the disruptive, unbridled diplomacy of special envoys that we can trace their genealogy back to the contested political arena of early postwar Japan.