Drawing on the conceptual framework of intermediations in grassroots innovation for sustainability, this paper presents the first in-depth analysis of the role of third sector organizations in citizen science. The empirical data are derived from 31 case studies of associations (representing 80% of administrative-type third sector organizations in France). We identify the features unique to the two blurry categories of observational science and social innovation: amateur community management is not confined to crowdsourcing, and legitimization of experiential knowledge is key to inclusion in co-creation processes. We also characterize the activities of theseassociations as research intermediations. The organizations fulfil three roles depending on the research field and the organization’s position in the network, the degree of structuration of their partnerships with academics, and the goals and achievements in the projects in which they are involved. These intermediations are not aimed solely at experimentation and advancement of a particular territory’s activities; they try also to influence the dominant system by producing knowledge and recommendations for public policies.