Why do women remain underrepresented in some science, technology, engineering, and math (STEM) fields? Research has uncovered various situational factors such as belongingness cues, but one understudied (and related) factor may be outperformance-related discomfort. Specifically, when people outperform others who are upset about being outperformed, they may experience sensitivity about being the target of a threatening upward comparison (STTUC; Exline and Lobel, in Psychological Bulletin 125:307–337, 1999). Three studies examined the potential role of STTUC among women in STEM and how STTUC relates to feelings of belongingness. Study 1 recruited a large internet sample of undergraduate women and found that the tendency to experience STTUC corresponded with relatively low levels of belongingness in both STEM and arts/humanities courses. Replicating prior research, results also showed significantly lower levels of belongingness in STEM vs. arts/humanities courses. Study 2 found higher anticipated levels of perceiving the outperformed person as upset and experiencing STTUC-related concerns among undergraduate women (vs. men) who imagined academically outperforming others. Study 3 examined women and non-binary first-year undergraduates in STEM and found that the tendency to experience STTUC corresponded to relatively low levels of belongingness in STEM at two time points. Taken together, results suggest that experiencing outperformance-related discomfort in STEM may predict relatively low levels of belongingness, which, over time, may contribute to weakening women’s interest in pursuing STEM. The present research may be of particular interest to educators, administrators, and policy makers interested in improving women’s representation in STEM.