Grassroots, bottom-up citizen science is a burgeoning form of public engagement with science, in which citizens mobilize scientific tools and data to address pressing real-world problems. Contrary to contributory (top-down) citizen science projects in which citizens collect data for professional scientists, these grassroots initiatives typically unfold in do-it-ourselves fashion alongside (or outside of) conventional approaches and channels, thereby challenging formally-sanctioned, expert-centric citizen science approaches and representations. This paper illustrates these points through a comparative analysis of two potentially paradigmatic sites for environmental grassroots citizen science glocally: Safecast (radiation pollution / Japan) and CuriousNoses (air pollution / Flanders, Belgium). Adopting a constructivist and relational account of grassroots citizen science as co-produced and shaped through collective practices, the paper draws out key features (defining moments, key actors, discourses, mechanisms, and devices) in the constitution of these networks as credible actors in affairs of environmental governance beyond the formally credentialed. The paper’s findings open onto a broader reflection on how to imagine citizen engagements in (grassroots) citizen science and the actual and potential roles social scientists assume as theorists, facilitators, and ‘interveners’ in these processes.