To determine the prevalence of rheumatic heart disease (RHD) in school-aged children and young people in Timor-Leste. Prospective cross-sectional survey. Echocardiography was performed by Australian cardiologists to determine the presence of RHD. Demographic data were also collected. Patients in whom RHD was detected were entered into a register to allow monitoring of adherence to secondary prophylaxis; the first dose of benzathine penicillin G (BPG) was administered on the day of screening. Schools in urban (Dili) and rural (Ermera) Timor-Leste. School students aged 5-20 years. Definite and borderline RHD, as defined by World Heart Federation echocardiographic criteria. 1365 participants were screened; their median age was 11 years (IQR, 9-14 years), and 53% were girls. The estimated prevalence of definite RHD was 18.3 cases per 1000 population (95% CI, 12.3-27.0 per 1000), and of definite or borderline RHD 35.2 per 1000 (95% CI, 26.5-46.4 per 1000). Definite (adjusted odds ratio [aOR], 3.5; 95% CI, 1.3-9.4) and definite or borderline RHD (aOR, 2.7; 95% CI, 1.4-5.2) were more prevalent among girls than boys. Eleven children (0.8%) had congenital heart disease. Of the 25 children in whom definite RHD was identified, 21 (84%) received education and a first dose of BPG on the day of screening; all 25 have since received education about primary care for RHD and have commenced penicillin prophylaxis. The rates of RHD in Timor-Leste are among the highest in the world, and prevalence is higher among girls than boys. Community engagement is essential for ensuring follow-up and the effective delivery of secondary prophylaxis.