Summary Obesity is a significant problem among adolescents in Pacific populations. This paper reports on the outcomes of a 3‐year obesity prevention study, Healthy Youth Healthy Communities, which was part of the Pacific Obesity Prevention in Communities project, undertaken with Fijian adolescents. The intervention was developed with schools and comprised social marketing, nutrition and physical activity initiatives and capacity building designed to reduce unhealthy weight, and the individual exposure period was just over 2‐year duration. The evaluation incorporated a quasi‐experimental, longitudinal design in seven intervention secondary schools near Suva (n = 874) and a matched sample of 11 comparison secondary schools from western Viti Levu (n = 2,062). There were significant differences between groups at baseline; the intervention group was shorter, weighed less, had a higher proportion of underweight and lower proportion of overweight, and better quality of life (Pediatric Quality of Life Inventory only). At follow‐up, the intervention group had lower percentage body fat (−1.17) but also a lower increase in quality of life (Assessment of Quality of Life instrument: −0.02; Pediatric Quality of Life Inventory: −1.94) than the comparison group. There were no other differences in anthropometry, and behaviours' changes showed a mixed pattern. In conclusion, this school‐based health promotion programme lowered percentage body fat but did not reduce unhealthy weight gain or influence most obesity‐promoting behaviours among Fijian adolescents. Despite growing evidence supporting the efficacy of community‐based approaches to reduce obesity among children of European descent, findings from this study failed to demonstrate the efficacy of a community capacity‐building approach among an adolescent sample drawn from a different sociocultural, economic and geographical context. Additional ‘top–down’ or other innovative approaches may be needed to reduce adolescent obesity in the Pacific.