Many college students intend to pursue science, technology, engineering, and math (STEM) careers but quickly abandon these goals when confronted with notoriously competitive STEM courses that often pit students against each other. This emphasis on interpersonal competition could be especially detrimental for first-generation (FG) college students, an underrepresented group in STEM fields which more strongly values communality and collaboration relative to their continuing-generation peers. Thus, FG students may experience more imposter feelings in STEM courses perceived as having a competitive culture. A longitudinal study (with 818 students and 2,638 experience-sampling observations) found that perceived classroom competition was associated with greater daily in-class imposter feelings among all students—but especially among FG students. Imposter feelings in turn predicted students’ end-of-term course engagement, attendance, dropout intentions, and course grades. Classroom competition and the imposter feelings it engenders may be an overlooked barrier for promoting the engagement, performance, and retention of FG students in STEM.