This study explores the Beheiren movement (also known as The Citizens’ Alliance for Peace in Vietnam), Japan’s first transnational anti-war movement (1965–1974). It focuses on the transcultural formation and transformation of its movement identity during the Vietnam war. Initially, movement participants developed an ethnoracial consciousness toward the Vietnamese based on their perceptions of a common victimization by U.S. imperialism. Yet, Beheiren leaders’ transcultural interactions with both Korean detainees in the Omura Detention Facility in Japan and members of the Black Power movement in the United States helped them to develop an antiracist consciousness toward fellow Asians beyond the context of the Vietnam war. While their interactions with Korean detainees forced them to examine Japan’s internal structural oppressions against fellow Asians (e.g., Korean residents in Japan) and fellow Japanese (e.g., the Okinawans), their interactions with the Black Power movement raised their awareness of antiracism and Third-Worldism. These interactions were necessary to transforming the movement’s identity, resulting in a confrontation with its own anti-Asian racism in the local cultural context of Japanese society as well as in the form of economic imperialism in Asia.