ABSTRACT The triple disaster in March 2011 (i.e. 3.11) triggered a national crisis that affected Japanese society on many levels. Strikingly, this crisis gave visibility to protest groups with no prior political activity organizing demonstrations against the government. One such group was Mama no Kai, an assembly of anti-war mothers protesting against peace and security legislation and the amendment of Article 9 of the Constitution of Japan. This paper focuses on Mama no Kai activism and the group’s use of motherhood discourses. Through participant observation and semi-structured interviews with assembly members, the study aims to identify the primary characteristics of mothers’ activism in the post-3.11 era, the development of the use of motherhood discourses, and the contribution of their strategies to the paradigm shift of mothers’ movements in Japanese urban activism.