Aim: To assess the effectiveness of a cognitive-behavioral treatment (CBT) program on weight reduction among Iranian adolescents who are overweight. Methods: Using a randomized controlled trial design, 55 adolescents who were overweight (mean [SD] age = 14.64 [1.69] years; zBMI = 2.18 [0.65]) were recruited in the CBT program and 55 in the treatment as usual (TAU; mean age = 14.88 [1.50]; zBMI = 2.09 [0.57]) group. All the participants completed several questionnaires (Child Dietary Self-Efficacy Scale; Weight Efficacy Lifestyle questionnaire; Physical Exercise Self-Efficacy Scale; Pediatric Quality of Life Inventory; and self-reported physical activity and diet) and had their anthropometrics measured (height, weight, waist and hip circumferences, and body fat). Results: The CBT group consumed significantly more fruits and juice, vegetables, and dairy in the 6-month follow-up as compared with the TAU group (p-values <0.001). The CBT group consumed significantly less sweet snacks, salty snacks, sweet drinks, sausages/processed meat, and oils in the six-month follow-up compared with the TAU group (p-values<0.001). Additionally, the waist circumference, BMI, waist-hip ratio, and fat mass were significantly decreased in the CBT group in the six-month follow-up compared with the TAU group (p-values<0.005). The CBT group significantly improved their psychosocial health, physical activity, and health-related quality of life (p-values<0.001). Conclusion: The CBT program showed its effectiveness in reducing weight among Iranian adolescents who were overweight. Healthcare providers may want to adopt this program to treat excess weight problems among adolescents.