Although social capital has been linked to a variety of health outcomes, its association with obesity has yet to be elucidated. This study explored the relations between social capital measured at the US state and county levels and individual obesity and leisure-time physical inactivity. Individual-level data were drawn from the 2001 Behavioral Risk Factor Surveillance System survey, while data from other surveys and administrative sources were used to construct contextual measures. Two state-level social capital scales were derived from 10 indicators and two county-level scales from five indicators. In 2-level analyses of over 167,000 adults nested within 48 states plus the District of Columbia, residence in a state above the median on one or both state social capital scales (vs. neither scale) was associated with lower relative odds of obesity and physical inactivity, controlling for individual-level covariables and state-level estimates of mean household income, the Gini coefficient, and the percentage of Black residents. In 3-level analyses, the adjusted odds ratio (OR) for physical inactivity associated with residence in a county above the median on one or both county social capital scales was significantly below 1, while the association with obesity was not significant. Significantly weaker inverse ORs for the relations between state- and county-level social capital and obesity were observed among American Indians/Alaska Natives compared to Whites. Meanwhile, little support was found for mediation by social capital of the associations of urban sprawl and income inequality with obesity or physical inactivity. Overall, this study provides some evidence for the promotion of social capital as a potential strategy for addressing the burgeoning obesity epidemic.