OBJECTIVE To evaluate the impact of a 12-month multicomponent school-based obesity prevention program, Nutrition and Enjoyable Activity for Teen Girls among adolescent girls. DESIGN Group randomized controlled trial with 12-month follow-up. SETTING Twelve secondary schools in low-income communities in the Hunter and Central Coast regions of New South Wales, Australia. PARTICIPANTS Three hundred fifty-seven adolescent girls aged 12 to 14 years. INTERVENTION A multicomponent school-based intervention program tailored for adolescent girls. The intervention was based on social cognitive theory and included teacher professional development, enhanced school sport sessions, interactive seminars, nutrition workshops, lunch-time physical activity sessions, handbooks and pedometers for self-monitoring, parent newsletters, and text messaging for social support. MAIN OUTCOME MEASURES Body mass index (BMI, calculated as weight in kilograms divided by height in meters squared), BMI z score, body fat percentage, physical activity, screen time, dietary intake, and self-esteem. RESULTS After 12 months, changes in BMI (adjusted mean difference, -0.19; 95% CI, -0.70 to 0.33), BMI z score (mean, -0.08; 95% CI, -0.20 to 0.04), and body fat percentage (mean, -1.09; 95% CI, -2.88 to 0.70) were in favor of the intervention, but they were not statistically different from those in the control group. Changes in screen time were statistically significant (mean, -30.67 min/d; 95% CI, -62.43 to -1.06), but there were no group by time effects for physical activity, dietary behavior, or self-esteem. CONCLUSIONS A school-based intervention tailored for adolescent girls from schools located in low-income communities did not significantly reduce BMI gain. However, changes in body composition were of a magnitude similar to previous studies and may be associated with clinically important health outcomes. TRIAL REGISTRATION anzctr.org.au Identifier: 12610000330044.