Nonsuicidal self-injury (NSSI) is a prevalent, concerning behavior among adolescents. Importantly, NSSI can serve a variety of functions. Some adolescents engage in NSSI to fulfill automatic or self-oriented functions (e.g., cutting to avoid internal negative states), whereas others engage in NSSI to serve social functions (e.g., cutting to communicate with others). This study tests whether self-reported reasons for engaging in NSSI, hereafter referred to as NSSI functions, predict NSSI thoughts and behaviors during and after hospitalization among adolescent psychiatric inpatients. Endorsement of both automatic and social NSSI functions, as well as positive and negative reinforcement subtypes, was assessed at hospital admission. Results showed that endorsement of overall automatic function predicted which adolescents engaged in NSSI behavior during hospitalization. Moreover, automatic and social functions showed distinct predictive patterns, such that automatic functions corresponded to greater likelihood of NSSI-related thoughts and behaviors whereas social functions mainly corresponded to reduced likelihood of NSSI-related outcomes. Of note, NSSI functions were less predictive of NSSI-related outcomes after hospital discharge. These findings suggest that identifying adolescent inpatients’ reasons for NSSI engagement may meaningfully distinguish those at higher risk (and those at lower risk) of NSSI persistence during their hospital stay.