INTRODUCTION: People who identify as sexual minorities (SM) report higher rates of nonsuicidal self-injury (NSSI) than heterosexual individuals. One explanatory factor that may be important in this relation is self-defective beliefs. The present study examined the role of self-defective beliefs in the link between sexual orientation and NSSI. METHOD: Participants were college-aged women (n = 145) with (n = 84) and without (n = 61) recent, recurrent NSSI, completed measures of NSSI and self-defective beliefs. Participants were recruited from a large college and surrounding area in the northeastern U.S. RESULTS: suggest that SM participants were more likely to report a history of NSSI, frequent and versatile NSSI, and greater self-defective beliefs compared to heterosexual participants, but not more likely to report medically severe NSSI. Results from cross-sectional mediation analyses revealed that self-defective beliefs mediated the relation between SM identity and lifetime NSSI history and partially mediated the relations for NSSI medical severity and versatility, but not NSSI frequency. CONCLUSIONS: Taken together, our findings suggest that self-defective beliefs are a salient cognitive concern associated with indicators of NSSI severity in college-aged SM women. Additional research is needed to determine whether these findings replicate in SM men, transgender, and gender diverse populations.