Who speaks for “citizen science” on Twitter? Which territory of citizen science have they made visible so far? This paper offers the first description of the community of users who dedicate their online social media identity to citizen science. It shows that Twitter users who identify with the term “citizen science” are mostly U.S. science professionals in environmental sciences, and rarely projects' participants. In contrast to the original concept of “citizen science”, defined as a direct relationship between scientists and lay participants, this paper makes visible a third category of individual actors, mostly women, who connect these lay participants and scientists: the “citizen science broker”.