This article explores and discusses understandings of citizen science with members of Japanese citizen radiation measuring organizations who began measuring radioactive contamination in food, soil, air, and human bodies after the 2011 Fukushima nuclear accident. Building on in-depth interviews with organization members and extensive multi-site fieldwork (2018, 2020), the article takes shimin kagaku (citizen science in Japanese) to examine articulations of citizenship and science, while discussing citizen radiation measuring organization activities. Adopting Tsing’s notion of nonscalability, it draws attention to the manifold articulations of citizenship and science, unearthing frictions embedded in Japanese science–society relations. In this way, this article outlines the diversity of notions of citizenship and science, and of citizen participatory practices in science. By bringing nonscalability to bear on an analysis of different articulations of shimin kagaku, this article encourages scientists and public authorities to engage with citizen participatory practices reflexively and responsibly by considering local articulations and knowledges.