Purpose: Both schools and neighborhoods play important roles in determining adolescent weight status, but little is known about their relative importance, particularly in predicting long-term weight outcomes. We assessed the impacts of both school and neighborhood socioeconomic composition, social connectedness, and built environment during adolescence on weight status in young adulthood. Methods: The study sample consisted of 14,625 respondents from Waves I and IV of the National Longitudinal Study of Adolescent to Adult Health. Data were analyzed using cross-classified multilevel modeling to examine the joint effect of adolescents’ school and neighborhood predictors on body mass index (BMI) 13 years later. Results: Living in a neighborhood with lower average parent education during adolescence, and attending a school with lower average parent education, were each associated with higher BMI in young adulthood. Living in a neighborhood with more physical activity resources predicted lower young adult BMI, independent of adolescent weight, parent obesity status, and demographic characteristics. School physical activity resources and perceptions of social connectedness (in the school or neighborhood) were not significantly associated with young adult BMI. Conclusions: These findings highlight the importance of school and neighborhood socioeconomic composition during adolescence on young adult weight status. Results also suggest that improving neighborhood infrastructure may promote healthy weight.