A growing interest in startups in the circular economy emerges which is perceived as one pathway to a sustainable development. Circular startups are concerned with the prolonging of resource use and the reduction of waste. For realizing their business models, collaborating with partners of different kinds is deemed crucial. As circular business models are rather new and lack in diffusion, circular startups face the challenge of persuading potential partners to be acknowledged as legitimate. This qualitative research sheds light on the cognitive frames applied by circular startups against this backdrop. We draw on 31 interviews with startups in an incubation program focused on circular economy and observed their attempts at gaining partners. We find that four frames are used individually and in combination in order to both legitimate their business model as well as the overarching concept of the circular economy. Our model shows how these cognitive frames are related to an overarching process of legitimation of their circular entrepreneurial action.