Background: School-based trials to prevent and reduce prevalence of pediatric obesity in low-income countries are necessary. In Brazil, addressing adolescent obesity is a public health priority. Objective: To evaluate the impact of a group randomized controlled trial involving a 6-month multicomponent school-based obesity prevention program targeting adolescent girls. Methods: The Healthy Habits, Healthy Girls-Brazil program recruited participants (n = 253; 16.05 ± 0.05 years) from ten eligible public technical schools in São Paulo, Brazil. The program was adapted from an Australian intervention study, which is based on the Social Cognitive Theory. The primary outcome measure was body mass index (BMI), and secondary outcomes included BMI z score, waist circumference, and various sedentary and dietary health-related behaviours. Results: Although changes in BMI were not statistically significant, differences favored the intervention group (adjusted mean difference, -0.26 kg/m²,se SE = 0.018, p = 0.076). Statistically significant intervention effects were found for waist circumference (-2.28 cm; p =, p = 0.01), computer screen time on the weekends (0.63 h/day, p; p = 0.02), total sedentary activities on the weekends (-0.92 h/day, p