To determine the efficacy of group cognitive-behavioral therapy (CBT) on adolescents with attention-deficit/hyperactivity disorder (ADHD) who were in pharmacological treatment but still had persistent symptoms. We conducted a multicenter, randomized, rater-blinded, controlled trial between April 2012 and May 2014 in a cohort of 119 adolescents (15-21 years of age). Participants were randomly assigned to 12 manualized group CBT sessions (n = 45) or a waiting list control group (n = 44). Primary outcomes were assessed by a blinded evaluator (ADHD Rating Scale ADHD-RS, Clinical Global Impression Scale for Severity CGI-S, Global Assessment of Functioning GAF) before and after treatment, as well as by self-report and parent informant ratings. Of the initial 119 participants enrolled, 89 completed treatment. A mixed-effects model analysis revealed that participants who were assigned to the group CBT sessions experienced significantly reduced ADHD symptoms compared to the control group (ADHD-RS Adolescent: -7.46, 95% CI = -9.56 to -5.36, p < .001, d = 7.5; ADHD-RS Parents: -9.11, 95% CI = -11.48 to -6.75, p < .001, d = 8.38; CGI-S Self-Report: -0.68, 95% CI = -0.98 to -0.39, p < .001, d = 3.75; CGI-S Clinician: -0.79, 95% CI = -0.95 to -0.62, p < .001; d = 7.71). Functional impairment decreased significantly in the CBT group according to parents (Weiss Functional Impairment Scale -4.02, 95% CI = -7.76 to -0.29, p < .05, d = 2.29) and according to the blinded evaluator (GAF: -7.58, 95% CI = -9.1 to -6.05, p < .001, d = 7.51). Group CBT associated with pharmacological treatment is an efficacious intervention for reducing ADHD symptoms and functional impairment in adolescents. Clinical trial registration information-CBT Group for Adolescents With ADHD: a Randomized Controlled Trial; http://clinicaltrials.gov/; NCT02172183.