Hybrid organizational forms combine values and practices from different institutional domains, rendering them difficult to fit neatly into the structures of extant organizational forms. Since the work required to institutionalize a new hybrid organizational form may be beyond the resources and capabilities of individual organizations acting alone, we shift the focus to inter-organizational collective action. Using empirical data from a study of a European network of renewable energy cooperatives, we find that, in order to institutionalize the new hybrid organizational form, the network can contribute to overcome the legitimacy challenges inherent in organizational hybridity. In particular, the network builds field-level receptivity to institutional pluralism, collectively codifies the hybrid organizational form, and consolidates legitimation towards plural field-level audiences. In order to perform these institutionalization roles, the network itself becomes increasingly formalized and mobilizes mediating functions involving different types of resources, legitimacy and target audiences. The research advances knowledge of hybrid organizational forms and their collective institutionalization through inter-organizational networks.