The prevalence of obesity among youths 2 to 19 years of age in the US from 2017 to 2018 was 19.3%; previous studies suggested that school lunch consumption was associated with increased obesity. The Healthy, Hunger-Free Kids Act of 2010 (HHFKA) strengthened nutritional standards of school-based meals. To evaluate the association between the HHFKA and youth body mass index (BMI). This cohort study was conducted using data from the Environmental Influences on Child Health Outcomes program, a nationwide consortium of child cohort studies, between January 2005 and March 2020. Cohorts in the US of youths aged 5 to 18 years with reported height and weight measurements were included. Full implementation of the HHFKA. The main outcome was annual BMI z-score (BMIz) trends before (January 2005 to August 2016) and after (September 2016 to March 2020) implementation of the HHFKA, adjusted for self-reported race, ethnicity, maternal education, and cohort group. An interrupted time-series analysis design was used to fit generalized estimating equation regression models. A total of 14 121 school-aged youths (7237 [51.3%] male; mean [SD] age at first measurement, 8.8 [3.6] years) contributing 26 205 BMI measurements were included in the study. Overall, a significant decrease was observed in the annual BMIz in the period following implementation of the HHFKA compared with prior to implementation (-0.041; 95% CI, -0.066 to -0.016). In interaction models to evaluate subgroup associations, similar trends were observed among youths 12 to 18 years of age (-0.045; 95% CI, -0.071 to -0.018) and among youths living in households with a lower annual income (-0.038; 95% CI, -0.063 to -0.013). In this cohort study, HHFKA implementation was associated with a significant decrease in BMIz among school-aged youths in the US. The findings suggest that school meal programs represent a key opportunity for interventions to combat the childhood obesity epidemic given the high rates of program participation and the proportion of total calories consumed through school-based meals.