Identity theory (IT) and social identity theory (SIT) are eminent research programs from sociology and psychology, respectively. We test collective identity as a point of convergence between the two programs. Collective identity is a subtheory of SIT that pertains to activist identification. Collective identity maps closely onto identity theory’s group/social identity, which refers to identification with socially situated identity categories. We propose conceptualizing collective identity as a type of group/social identity, integrating activist collectives into the identity theory model. We test this conceptualization by applying identity theory hypotheses to the “vegan” identity, which is both a social category and part of an active social movement. Data come from comments on two viral YouTube videos about veganism. One video negates prevailing meanings of the vegan identity. A response video brings shared vegan identity meanings back into focus. Identity theory predicts that nonverifying identity feedback elicits negative emotion and active behavioral response, while identity verification elicits positive emotion and an attenuated behavioral response. We test these tenets using sentiment analysis and word counts for comments across the two videos. Results show support for identity theory hypotheses as applied to a collective social identity. We supplement results with qualitative analysis of video comments. The findings position collective identity as a bridge between IT and SIT, demonstrate innovative digital methods, and provide theoretical scaffolding for mobilization research in light of emergent technologies and diverse modes of activist participation.