Research on user innovation shows that tensions in collaborations between firms and innovation communities can hinder innovation, and that innovation intermediaries can help resolve these tensions by bridging opposing interests. Despite the compelling role of innovation intermediaries, few studies on such mediation exist. Using an embedded case study, this article examines the role of an innovation intermediary that facilitates online innovation contests for client firms and identifies an apparent membership paradox evolving around three key tensions of power, competence, and identity. The article reveals that innovation intermediaries shape new understandings of power, competence, and identity that shift focus from resolving tensions to managing paradoxes. The membership paradox (re)appears at both the project level (between control and openness) and at the individual level, between professionalism and personality for employees and between imitation and authenticity for community participants. This article contributes to the user innovation literature by demonstrating how opposing firm and community interests are mediated through managing new forms of membership uncertainty. Moreover, the lens of paradox management offers a novel dimension to explaining why tensions that arise between firms and innovation communities are difficult to resolve, and also how the ensuing gaps in mutual understanding might be tackled. Theoretical and managerial implications of these findings for user innovation researchers and practitioners are discussed.