This study tested the efficacy of an Internet-based health promotion program, BodiMojo, designed to promote positive body image in adolescents. Participants were 178 students (mean age 15.2 years, 67.6% ethnic minority) in three public high schools. Intervention groups used BodiMojo for four weekly health class periods, while controls participated in their usual health curriculum. Body image measures were given at baseline, post-intervention, and 3 months. Girls reported decreased body dissatisfaction (p<.05), decreased physical appearance comparison (p<.05), and increased appearance satisfaction (p<.05), relative to controls. Effects were not maintained at 3-month follow-up. No significant differences were found between the intervention and control groups with boys. Moderation analyses suggested positive effects for diverse adolescents as well as those who were overweight or indicated baseline high body dissatisfaction. BodiMojo appears to be modestly effective in decreasing body image concerns among adolescent girls in the short term.