Recent shifts in masculinity norms have led many men to adopt roles traditionally associated with femininity. This research examined whether perceived changes in gender norms influence heterosexual men’s willingness to intervene in workplace sexual harassment. Across three experimental studies (N = 614) in the United Kingdom, United States, and Western Europe, masculinity norms were manipulated (perceived feminization vs. traditional masculinity) and antifemininity motivation was assessed. Participants then evaluated a workplace sexual harassment scenario and reported willingness to intervene. Results indicated that perceived feminization reduced willingness to intervene (Studies 1–3) and increased justification of harassment (Study 3) among men high in antifemininity motivation. These findings highlight the role of masculinity norms in shaping bystander intentions and suggest that shifts toward inclusive masculinity may have unintended consequences for prosocial responses to sexual harassment.