Political efficacy-the belief that one can influence politics-is a key predictor of people's involvement in social movements. Political institutions that are open to change should, however, be seen as just. Thus, political efficacy may ironically undermine minority group members' support for collective action by simultaneously increasing their belief in the fairness of the system. The current study aims to examine this possibility in a national sample of Māori-New Zealand's indigenous minority population. Participants (N = 399) were Māori (Mage = 44.22; SD = 13.30) women (n = 272) and men (n = 115; unreported = 12) who completed a survey assessing their levels of (a) political efficacy, (b) system justification, and (c) support for the political mobilization of their group, as well as relevant demographic covariates. Consistent with past research, political efficacy had a positive direct effect on participants' support for the political mobilization of Māori. Nevertheless, political efficacy also had a negative indirect effect on political mobilization support via increases in system justification. These results held after controlling for participants' ethnic identification, self-efficacy, and conservatism. Our findings uncover a hidden consequence of political efficacy and show that, while believing that the political system is receptive to change predicts political mobilization, it can also undermine minorities' support for the mobilization of their group. Thus, our results uncover a previously unknown process that maintains inequality between ethnic minority and majority group members.