Although much is known about why people engage in collective action participation (e.g., politicized identity, group-based anger), little is known about the psychological consequences of such participation. For example, can participation in collective action facilitate attitude moralization (e.g., moralize their attitudes on the topic)? Based on the idea that collective action contexts often involve a strong social movement fighting against an immoral adversary, we propose that participating in collective action facilitates attitude moralization over time. By integrating the moralization and collective action literatures, we hypothesized that participation in collective action moralizes individuals' attitudes over time because it politicizes their identity, enrages them vis-a-vis the outgroup, and/or empowers them to achieve social change. We tested these hypotheses in a 2-year, five-wave longitudinal study (N = 1,214) in the contentious context of the Chilean student movement. We examined within-person (and between-person) changes over time and consistently found that participation in collective action predicted individual changes in moral conviction over time through politicized identification and group-based anger toward the outgroup. Furthermore, moral conviction predicted participation in collective action over time-an effect consistently explained by politicized identification. These findings are the first to show that (a) participation in collective action moralizes individuals' attitudes because it politicizes their identity and enrages them vis-a-vis the (immoral) outgroup and that (b) moralization in turn helps to better understand sustained movement participation. Theoretical implications for the literature on moralization and collective action are discussed.