Previous research has indicated that witnessing gender discrimination may instigate women’s participation in collective action for gender justice. However, relatively little is known about the role of perceived female support in motivating collective action among women who witness gender discrimination in public life. This study aims to analyse whether and when perceived support from feminist-minded women moderates the association between women’s witnessing gender discrimination and their willingness to engage in collective action for gender justice. We argue that the association between witnessing gender discrimination and willingness to engage in collective action depends on the support women perceive from their female friends and family members. In studies of women in the U.S. (Study 1; N = 271) and Ukraine (Study 2; N = 256), witnessing gender discrimination predicted greater willingness to participate in collective action for gender justice, and this association was stronger when female support was perceived to be lower. Study 3 (N = 1,304) replicated the findings of Studies 1 and 2 with self-identified feminist women in Turkey. Our research offers novel insights regarding why perceived lack of female support may encourage women to engage in collective action for gender justice.