This mixed-methods study examines the experience of the imposter phenomenon in a racially/ethnically and sexually diverse sample of undergraduates in majors related to science, technology, engineering, and math (STEM). Guided by an intersectionality framework, we examined whether experiences of the imposter phenomenon differ at the intersection of race/ethnicity and sexual identity. Survey data from undergraduate STEM students (N = 735) consisting of 559 (76%) racial/ethnic minorities and 169 (23%) lesbian, gay, bisexual, transgender, and queer/questioning individuals revealed racial/ethnic and sexual identity differences in the amount of imposter phenomenon. Findings also suggest that STEM students with more mentors reported lower imposter phenomenon. This finding was not moderated by race/ethnicity or sexual identity, implying that mentoring may be a form of social support among students in STEM education. To supplement the quantitative findings, we collected open and close-ended data from participants. Thematic analysis of these data provides insight into the range of coping strategies STEM students use when experiencing the imposter phenomenon. Implications and recommendations for research, theory, and policy are further discussed.