Background: Social capital in neighborhoods and workplaces positively affects health. Less is known about the influence of school social capital on student health outcomes, in particular weight status. We sought to examine the association between individual- and school-level social capital and student body mass index (BMI). Methods: Analyzing data from the National Longitudinal Study of Adolescent Health, a nationally representative sample of adolescents in grades 7-12 (N = 13,428), we used principal components analysis to define 3 school social capital factors: "connectedness" (feel part of/close to people/safe in school), "treatment" (get along with teachers/students, teachers treat students fairly), and "parental involvement" (school administrator reported percent family/parent self-reported participation in Parent Teacher Organization, average daily school attendance). We examined the associations between individual- and school-level social capital and individual BMI using multilevel modeling techniques. Results: In girls, both feeling connected to one's school (ß = -0.06, p < 0.05) and attending schools with overall high connectedness (b = -0.43, p < 0.01) were associated with lower BMIs. In boys only attending a school with high "treatment" was inversely associated with BMI (b = -0.61, p < 0.01), adjusting for individual and school demographics. Conclusions: Although further studies are needed, our findings suggest enhancing school social capital as a novel approach to addressing student obesity.