Background: This study evaluated the impact of a school-based nutrition program on preventing overweight and obesity in children in Thailand from 2014 to 2019. Methods: We used difference-in-differences with matched comparison group to evaluate program impact. Treatment schools were categorized into those that participated in phase 1 only, and those that participated in phase 1 and 2. We matched 311 treatment schools in phase 1 only with 1504 comparison schools, and 75 treatment schools in both phases with 216 comparison schools. Administrative data from Thailand Office of Basic Education Commission were used from 2014 (baseline), 2016 (first follow-up), and 2019 (second follow-up). Results: Program resulted in a 0.6-1.1 percentage point reduction in the students' probability of overweight by the end of phase 1, and 1.7 percentage points by the end of phase 2. Impact on obesity was found only for schools participating in both phases (0.4 and 0.9 percentage points by the end of phase 1 and 2, respectively). Implications For School Health Policy, Practice, and Equity: Program continuity and intensity are key to achieve a reduction in overweight and obesity in schoolchildren. Conclusions: The program was successful in reducing overweight and/or obesity given appropriate level of continuity and intensity.