RESUMEN
Negative body image is a common public health concern among adolescents, globally. The aim of the current study was to evaluate the effectiveness, implementation fidelity, and acceptability of a single session, school-based universal body image intervention in Indonesia. A total of 1926 adolescents (59.4 % girls) and 12 school guidance counsellors (lesson facilitators) from nine state junior secondary schools in Surabaya, East Java took part in a two-arm open parallel cluster randomised controlled trial. In response to the changing circumstances due to the COVID-19 pandemic, half of the lessons were conducted in person and half were delivered online. Results showed that the lesson did not significantly improve adolescent body image or secondary outcomes relative to the control, though there was no evidence of harm. There were no substantive findings regarding intervention effectiveness by gender. The mode of intervention delivery (online vs. in-person) did not significantly influence the main findings. Implementation fidelity varied widely, and the lesson content and pedagogy were largely acceptable, though there was a strong preference for in-person lesson delivery. Findings have implications for researchers aiming to improve adolescent body image in low- and middle-income countries. Lessons learned can inform future school-based efforts to support adolescent body image.