Campaigns supporting victims of gender harassment and abuse, such as #MeToo, have made and still make significant contributions to achieving the fifth UN Sustainable Development Goal aimed at eliminating “all forms of violence against all women”. In two correlational studies, we examined possible antecedents of people’s willingness to participate in the #MeToo campaign by focusing on the role of ambivalent beliefs toward women and men and the perceived effects of the movement. Men (Study 1) and women (Study 2) were asked to answer questions concerning hostile and benevolent beliefs about women and men, respectively, their perception of the beneficial and detrimental effects of #MeToo, and their intentions to participate in the campaign. Study 1 showed that men’s hostile sexism toward women was associated with fewer intentions to actively support the #MeToo campaign and that the reduced beliefs that the movement had beneficial effects mediated this relation. Study 2 revealed that women’s stronger benevolent beliefs about men were associated with decreased perception that the #MeToo campaign had a beneficial impact. In turn, such a perception was related to lower intentions to participate in supporting the campaign. Moreover, women’s hostility toward men explained the intention to join the #MeToo movement through the mediation of the perception that the campaign was beneficial. The findings suggest that to foster participation in a feminist movement that promotes women’s rights, it is necessary to eradicate traditional gender roles and the related ideologies that legitimate men’s dominant position in society.