Abstract: This study illustrates how citizen-driven radiation monitoring has emerged in post-Fukushima Japan, where citizens generate their own radiation data and measurement devices to provide public with actionable data about their environments. Drawing on ethnographic fieldwork in and around Fukushima Prefecture, it highlights the multifaceted character of these bottom-up, citizen-led efforts, contrasting these initiatives with the emergence of “citizen participatory” science policy discourses in Japan. Recognizing the contested nature of citizenship in Japan and in the nuclear arena, the article considers how terms and definitions shape the participation of citizens and other stakeholders (local communities, public authorities, regulators, and professional scientists) in science and technology in culturally and historically specific ways. It builds on these observations to open up new spaces of expertise, which engage all stakeholders through social-scientific intervention.