Impostor phenomenon or “impostorism” refers to the experience of high-achieving individuals (particularly women) who, despite being successful, attribute their accomplishments to luck, and fear being exposed as frauds. In the current study, we examined the association between impostorism and graduate student self-efficacy, perceptions of the research-training environment, and attitudes toward academic persistence of 224 women completing a science, technology, engineering, and mathematics (STEM)-related doctoral degree. As hypothesized, participants who identified more strongly with impostor characteristics reported a lower sense of self-efficacy, more negative views of their academic context, and more pessimistic outlooks toward attaining their doctorate. However, results from a multiple mediation analysis revealed that women’s levels of self-efficacy and perceptions of their doctoral environment accounted for the effects of impostorism on their attitudes about academic persistence. Also, the relation between impostorism and persistence attitudes strengthened as numerical representation of women in a STEM program increased. Results illuminate the potential role STEM departments can have on students’ persistence by developing early opportunities for research collaborations and fostering an early sense of accomplishment. Parents and teachers might also draw from our findings to develop strategies to inoculate younger students from the insidious effects of gender-based stereotypes. Online slides for instructors who want to use this article for teaching are available on PWQ's website at http://journals.sagepub.com/page/pwq/suppl/index