Building on intergroup emotion research, we test the idea that intergroup emotion influences self-categorization. We report two studies using minimal (Study 1) and natural (Study 2) groups in which we measured participants' emotional reactions to a group-relevant event before manipulating the emotional reactions of other ingroup members and outgroup members (anger vs. happiness in Study 1; anger vs. indifference in Study 2). Results supported the hypotheses that (a) the fit between participants' own emotional reactions and the reactions of ingroup members would influence self-categorization, and (b) the specific content of emotional reactions would shape participants' willingness to engage in collective action. This willingness was greater when emotional reactions were not only shared with other group members, but were of anger (consistent with group-based action) rather than happiness or indifference (inconsistent with group-based action). Implications for the relationship between emotion and social identities are discussed.