Male-dominated work environments often possess masculine cultures that are unwelcoming to women. The present work investigated whether male-dominated academic environments were characterized by gender ideologies with negative implications for women. A survey of 2622 undergraduates across a variety of academic majors examined how gender imbalance within the major corresponded with students’ gender ideologies. We hypothesized that men in male-dominated domains might justify their dominance and prototypical status by adopting gender ideologies and stereotypes that denigrate women and treat men as the normative and superior group. Confirming this hypothesis, men in increasingly male-dominated academic majors were more likely to endorse Assimilationism—that women should adapt and conform to masculine work norms in order to succeed—and Segregationism—that men and women should pursue traditional social roles and careers. Moreover, they were less likely to endorse Gender Blindness—that attention to gender should be minimized. They were also more likely to agree with the gender-science stereotype that men do better in math and science than women. In contrast, gender imbalance in the major did not influence women’s gender ideologies, and women in increasingly male-dominated majors were significantly less likely to endorse the gender-science stereotype.