Group cognitive behavioral therapy (CBT) is an effective treatment for adolescent depression, but outcomes vary. Our goal was to examine interpersonal factors that predict response to group CBT for adolescent depression using a broad range of outcomes, including depressive symptoms, session attendance, treatment completion, engagement, and improvement. Seventy adolescents (age 14–18) with depression completed self-report measures of social support and parental conflict and were offered an established 16-session group CBT program. Correlation and regression analyses were conducted for interpersonal predictors and CBT outcomes. Accounting for pre-treatment depressive symptoms, fewer social supports predicted lower likelihood of finishing treatment and less clinician-rated improvement. Greater pre-treatment parental conflict predicted fewer sessions attended, lower clinician-rated engagement, and less clinician-rated improvement. Results highlight the need to consider interpersonal difficulties in CBT, as they may present a barrier to treatment attendance, engagement, and improvement.