In comparison with their peers, adolescents with ADHD are at increased risk for developing depression, with prevalence estimates for comorbid depression ranging from 14% to 20%. Youth with comorbid ADHD and depression are at greater risk for multiple negative outcomes compared to youth with ADHD alone, including suspension from school, failing a grade, or difficulties in peer relationships. Identifying risk factors for depression among adolescents with ADHD is important for facilitating early identification and treatment efforts. Research completed to date indicates that interpersonal impairment and emotion regulation may mediate the relationship between ADHD and depression. However, a comprehensive longitudinal model including both interpersonal impairment and emotion regulation has not been tested. The aim of the current study was to evaluate whether the indirect pathway from ADHD to depression through interpersonal impairment is moderated by emotion regulation in a longitudinal study of young adolescents with ADHD (N = 239; M age at follow-up = 12.30, SD = .92). Data were collected at three time points over 18 months. Parents completed ratings of externalizing symptoms (ADHD and Oppositional Defiant Disorder, [ODD]), parent-child relationship problems, and peer relationship problems. Adolescents rated their depressive symptoms and emotion regulation problems. Moderated mediation models as outlined by Preacher, Rucker, and Hayes (2007) were used to test whether the indirect effects of externalizing symptoms on depression through parent-child relationship stress and peer relationship problems are moderated by emotion regulation. Controlling for baseline levels of depression, results suggest that these pathways vary as a function of emotion regulation, such that the paths from externalizing symptoms to depression are significantly stronger among adolescents with high levels of emotion regulation problems. In addition, an exploratory analysis of the associations between multiple aspects of emotion regulation problems and depression revealed that access to emotion regulation strategies was the only unique predictor of later depression among adolescents with ADHD.