This article sheds light on the understudied significance of Islam, Communism, and global politics in defining what constituted an acceptable “religion” (shūkyō 宗教) in wartime Japan. An analysis of the Japanese Imperial Diet’s debates on the place of Islam in the Religious Organizations Law of 1939, which defined state-sanctioned religious organizations, reveals that Muslim attention from around the world, international politics, the global spread of Communism, and the relatively short history of Islam in Japan, affected politicians’ decision not to mention Islam as a religious organization in the law. While previous literature on the Religious Organizations Law has not adequately addressed the significance of international and non-Euro-American transnational influences, this article argues that lawmakers viewed the power of transnational Muslim and Communist networks as crucial when defining both officially acceptable “religion” and the Shrine (jinja 神社), or Shrine Shinto, as the national core to be protected under this law. The debates surrounding Islam offer fertile ground for examining the significance of global affairs in determining acceptable forms of “religion” in Japan, as well as the broader implications of what Japanese state officials called “religion” and “thought” (shisō 思想) in wartime Japanese and world politics.