This chapter traces the development of Public Health Demography, a field of population science represented by activities at the Department of Public Health Demography at the Institute of Public Health. The department was established in 1949 by Koya Yoshio, the Institute’s Director and leading wartime racial hygienist who became a birth control activist after the war. Drawing on existing work that locates Japanese birth control advocacy in transnational histories, the chapter argues that domestic efforts to discipline reproductive bodies within Japan, realized by population scientists such as Koya, were directly linked to collaborative working relationships with international colleagues to restrict world population growth by popularizing contraceptive practices in so-called underdeveloped nations, through development aid programs. At the same time, going beyond the existing literature, I also depict how the transnational movement fostered inter-Asian scientific interactions between Japanese and Indian colleagues via the funding support of the American Foundations, most notably the Population Council. Ultimately, this chapter portrays the Japanese state’s efforts to regulate citizens’ fertilities as a complex practice based on the co-production of scientific knowledge, scientific discipline and social order involving multi-layered interactions at local, national, regional and transnational levels.