Throughout Europe and North America, mainstream political parties have ceded electoral support to antiestablishment parties from the far left and far right. We investigate the hypothesis that individual differences in system justification—the psychological tendency to defend and justify the overarching social system—would be negatively associated with antiestablishment voting, even among citizens who would otherwise be inclined to support radicalism. In three large, nationally representative surveys conducted in France (N = 14,432), Germany (N = 1,168), and the United Kingdom (N = 2,337), we observed that system justification was positively associated with voting for establishment parties and negatively associated with antiestablishment voting. System justification was associated with reduced support for antiestablishment parties on the right and left—even among respondents who were high on ethnic intolerance, opposition to the European Union, economic distress, and support for income redistribution. Thus, all other things being equal, system‐justification tendencies reinforce political moderation, establishment voting, and therefore social stability.