This article examines how social movement actors can forge and sustain a collective identity despite heterogeneous backgrounds and the absence of pre-existing commonalities and networks. Based on an ethnography of the French yellow vest movement, we build on the concept of reactive identity to describe two key mechanisms. First, we show this movement’s collective identity crystallized through the actors’ shared reactions to the broader sociopolitical environment. Then, we describe how identification processes are reinforced when social movement actors feel rejected, stigmatized, and repressed in their interactions with national institutions, civil society, and individuals. We explain how these mechanisms are useful for understanding the development of collective identities within mass movements, which encompass individuals with various and fragmented identities. Exploring new dimensions of reaction beyond the us-versus-them mechanisms of identity formation, we show how collective identity can coalesce for groups who became stigmatized as they mobilize to oppose their environment.