OBJECTIVE: Our goal was to determine if racial/ethnic disparities in adolescent boys' and girls' physical activity participation exist and persist once the school attended is considered. METHODS: We performed a cross-sectional analysis of 17,007 teens in the National Longitudinal Study of Adolescent Health. Using multivariate linear regression, we examined the association between adolescent self-reported physical activity and individual race/ethnicity stratified by gender, controlling for a wide range of sociodemographic, attitudinal, behavioral, and health factors. We used multilevel analyses to determine if the relationship between race/ethnicity and physical activity varied by the school attended. RESULTS: Participants attended racially segregated schools; approximately 80% of Hispanic and black adolescent boys and girls attended schools with student populations that were <66% white, whereas nearly 40% of the white adolescents attended schools that were >94% white. Black and Hispanic adolescent girls reported lower levels of physical activity than white adolescent girls. There were more similar levels of physical activity reported in adolescent boys, with black boys reporting slightly more activities. Although black and Hispanic adolescent girls were more likely to attend poorer schools with overall lower levels of physical activity in girls; there was no difference within schools between black, white, and Hispanic adolescent girls' physical activity levels. Within the same schools, both black and Hispanic adolescent boys had higher rates of physical activity when compared with white adolescent boys. CONCLUSIONS: In this nationally representative sample, lower physical activity levels in Hispanic and black adolescent girls were largely attributable to the schools they attended. In contrast, black and Hispanic males had higher activity levels than white males when attending the same schools. Future research is needed to determine the mechanisms through which school environments contribute to racial/ethnic disparities in adolescent physical activity and will need to consider gender differences in these racial/ethnic disparities.