Objective: Social norms approaches to sexual assault prevention have proliferated despite a dearth of empirical evidence for the tenets of social norms theory as it relates to sexual assault. Whereas previous research has found that men’s perceptions of peer aggression influence their perpetration of sexual assault, previous research has not assessed the extent to which men’s perceptions are accurate about their close peers. Method: Undergraduate men (N = 100) from the psychology participant pool completed surveys along with a close friend (N = 100); the concordance in their beliefs about rape and attitudes toward women as well as reported sexually aggressive behaviors was assessed. Results: Men’s own beliefs about rape and attitudes about women were correlated with both their perceptions of their friends’ and of the average college male’s beliefs, but not with their friends’ actual reported beliefs; men’s perceptions of their friends’ beliefs about rape and attitudes toward women were uncorrelated with their friends’ actual reported beliefs as well. Perpetrators of sexual assault were significantly more likely to overestimate their friends’ involvement in sexually aggressive behaviors than were nonperpetrators. The order of measures presented was unrelated to endorsement of any of the variables of interest. Conclusions: Perpetrators of sexual assault hold inaccurate beliefs about their peers’ sexually aggressive attitudes and behaviors, which can be targeted in prevention programming. Such programming should provide more accurate descriptive (e.g., rates of sexual assault among men) as well as injunctive norms (i.e., rates of men’s approval or disapproval of attitudes and beliefs) to combat pluralistic ignorance and the false consensus effect. (PsycINFO Database Record (c) 2016 APA, all rights reserved)