Nonsuicidal self-injury (NSSI) is associated with increased risk of suicide attempts. Theories of NSSI assert interpersonal stress as a common risk factor for, and perhaps consequence of, NSSI. Prior research has not examined reciprocal associations between chronic interpersonal stress and NSSI. This study used a multiwave, prospective design to address this gap in a sample of adolescent girls, a group with elevated risk for both chronic interpersonal stress and NSSI. Pubertal development was examined as a moderator of the reciprocal associations.Adolescent girls (N = 220; ages 12-16, M age = 14.69 years) at heightened risk for NSSI completed a baseline assessment and follow-up assessments over 18 months, divided into two 9-month epochs (Time 1 and 2). Pubertal development was assessed via self- and parent-report. Chronic interpersonal stress was assessed using a semistructured interview at the end of each time period. NSSI was measured using a semistructured clinical interview every 3 months within both time periods to enhance accurate reporting.Path models revealed that chronic romantic stress during Time 1, but not peer or parent-child stress, predicted NSSI during Time 2 among girls with more advanced pubertal development. Moreover, NSSI during Time 1 predicted higher levels of chronic romantic and parent-child stress during Time 2.Results revealed a reciprocal relationship between chronic romantic stress and engagement in NSSI. Further, this association may be best understood in the context of pubertal development.