‘It's Your Move!’ was a 3-year intervention study implemented in secondary schools in Australia as part of the Pacific Obesity Prevention In Communities Project. This paper reports the outcome results of anthropometric indices and relevant obesity-related behaviours. The interventions focused on building the capacity of families, schools and communities to promote healthy eating and physical activity. Baseline response rates and follow-up rates were 53% and 69% respectively for the intervention group (n = 5 schools) and 47% and 66% respectively for the comparison group (n = 7 schools). Statistically significant relative reductions in the intervention versus comparison group were observed: weight (−0.74 kg, P < 0.04), and standardized body mass index (−0.07, P < 0.03), and non-significant reductions in prevalence of overweight and obesity (0.75 odds ratio, P = 0.12) and body mass index (−0.22, P = 0.06). Obesity-related behavioural variables showed mixed results with no pattern of positive intervention outcomes. In conclusion, this is the first study to show that long-term, community-based interventions using a capacity-building approach can prevent unhealthy weight gain in adolescents. Obesity prevention efforts in this important transitional stage of life can be successful and these findings need to be translated to scale for a national effort to reverse the epidemic in children and adolescents.