People are more willing to engage in collective action when they feel anger about collective disadvantage (van Zomeren, Postmes, & Spears, 2008). However, anger is often a fleeting emotion, whereas collective action is deliberate and sustained. Moreover, people cannot always engage in collective action on demand. We propose that rumination, or repetitive thoughts about negative emotions and experiences, may constitute a mechanism by which anger about collective disadvantage is sustained, and through it the motivation to engage in collective action. In three separate samples (128 students of color, 155 women, and 150 sexual minorities), we measured group-based anger about discrimination, group efficacy, group identification, rumination about discrimination, and collective action intentions. In all samples, rumination mediated the unique association between group-based anger and collective action intentions, controlling for group efficacy and group identification. Further, in an experimental study with 117 undergraduate women, we found that rumination, compared to distraction, sustained anger about collective disadvantage over a short period of time, and this sustained level of anger mediated the relationship between rumination and collective action intentions. These results lend support to conceptualizing collective action as a form of coping with collective disadvantage, and highlight the potential role of emotion regulation strategies like rumination in understanding intergroup relations.