Despite growing diversity among life sciences professionals, members of historically underrepresented groups (e.g., women) continue to encounter barriers to academic and career advancement, such as subtle messages and stereotypes that signal low value for women, and fewer opportunities for quality mentoring relationships. These barriers reinforce the stereotype that women's gender is incompatible with their science, technology, engineering, and mathematics (STEM) field, and can interfere with their sense of belonging and self-efficacy within STEM. The present work expands this literature in two ways, by 1) focusing on a distinct period in women's careers that has been relatively understudied, but represents a critical period when career decisions are made, that is, graduate school; and 2) highlighting the buffering effect of one critical mechanism against barriers to STEM persistence, that is, perceived support from advisors. Results of the present study show that perceived support from one's advisor may promote STEM engagement among women by predicting greater gender--STEM identity compatibility, which in turn predicts greater STEM importance among women (but not men). STEM importance further predicts higher sense of belonging in STEM for both men and women and increased STEM self-efficacy for women. Finally, we describe the implications of this work for educational policy.