To identify student- and school-level sociodemographic characteristics associated with overweight and obesity, the authors conducted cross-sectional analyses of data from 624,204 public school children (kindergarten through 12th grade) who took part in the 2007-2008 New York City Fitnessgram Program. The overall prevalence of obesity was 20.3%, and the prevalence of overweight was 17.6%. In multivariate models, the odds of being obese as compared with normal weight were higher for boys versus girls (odds ratio (OR) = 1.39, 95% confidence interval (CI): 1.36, 1.42), for black (OR = 1.11, 95% CI: 1.07, 1.15) and Hispanic (OR = 1.48, 95% CI: 1.43, 1.53) children as compared with white children, for children receiving reduced-price (OR = 1.17, 95% CI: 1.13, 1.21) or free (OR = 1.12, 95% CI: 1.09, 1.15) school lunches as compared with those paying full price, and for US-born students (OR = 1.54, 95% CI: 1.50, 1.58) as compared with foreign-born students. After adjustment for individual-level factors, obesity was associated with the percentage of students who were US-born (across interquartile range (75th percentile vs. 25th), OR = 1.10, 95% CI: 1.07, 1.14) and the percentage of students who received free or reduced-price lunches (across interquartile range, OR = 1.13, 95% CI: 1.10, 1.18). The authors conclude that individual sociodemographic characteristics and school-level sociodemographic composition are associated with obesity among New York City public school students.