Objective: To evaluate a commercially available, structured short-term weight management program designed for adolescents with obesity delivered by nonhealth professionals. Study design: A multisite parallel-group randomized controlled trial was conducted to evaluate a commercial 12-week lifestyle behavioral program in commercial weight management centers in Australia. Eligible participants (13-17 years, body mass index (BMI) z score ≥1.282 with no presenting morbidities) were randomized (n = 88) to intervention or wait-list, and the program was delivered by consultants at participating weight management centers. The primary outcome was change in BMI z score. Secondary outcomes included the psychometric variables quality of life, body-esteem, and self-esteem. Data was analyzed according to intention-to-treat principles. Results: Of 74 participants who consented to enter the study, 66 provided baseline anthropometric data and 12-week data were available for 55 individuals (74%). A significantly greater decrease in BMI z score in the intervention group (n = 32) was observed when compared with the wait-list control group, mean difference (MD) = -0.27 kg/m2; 95% CI, -0.37,-0.17; P < .001). Participants allocated to receive the lifestyle intervention reported a greater improvement in body esteem (MD = 1.7, 95% CI, 0.3, 3.1; P = .02) and quality of life (MD = 5.9, 95% CI, 0.9, 10.9; P = .02) compared with the wait-list control group. Conclusions: A structured lifestyle intervention delivered by a commercial provider in an adolescent population can result in clinically relevant weight loss and improvements in psychosocial outcomes in the short term. Further research is required to evaluate long-term outcomes. Trial registration: International Clinical Trials Registry: ISRCTN13602313.