Objective This study investigated the association of residential density with overweight among adolescents in an urban area of China. Methods Using multistage proportional sampling methods, a population-based cross-sectional study was conducted in Nanjing between September and November 2004 (n=2375; mean age=13.961.0 years old; 46.2% boys; survey response rate=89.3%). Body mass index was calculated from self-reported body weight and height. Overweight, the main outcome variable, was defined as a BMI ≥85 percentile value for age- and gender-specific reference data according to the recommendation for Chinese adolescents. The primary explanatory variable was the residential density of the urban districts. Mixed-effects logistic regression models were used for the analysis. Results Students in the higher and middle tertiles of residential density had a 2.17-fold (95% CI 1.41 to 3.33) and 1.89-fold (95% CI 1.22 to 2.92) higher likelihood of being overweight, respectively, compared with those in the lower tertile. The associations were slightly attenuated but still significant after adjusting for time spent in recreational physical activity and sedentary behaviour (viewing TV and sitting for academic study). Conclusions Residential density was positively associated with overweight among urban Chinese adolescents. Our findings warrant further research examining attributes of urban environments associated with adolescents' obesity in China and potential mechanisms between them.