Objective To estimate the prevalence of obesity in 5 major racial/ethnic groups in 4-year-old US children. Design Cross-sectional secondary data analysis. Setting Nationally representative sample of US children born in 2001. Participants Height and weight were measured in 2005 in approximately 8550 children who participated in the Early Childhood Longitudinal Study, Birth Cohort. Main Exposure Racial/ethnic group. Outcome Measure Prevalence of obesity, defined as body mass index at or above the 95th percentile for age of the sex-specific Centers for Disease Control and Prevention growth charts. Results Obesity prevalence among 4-year-old US children (mean age, 52.3 months) was 18.4% (95% confidence interval [CI], 17.1%-19.8%). Obesity prevalence differed by racial/ethnic group (P < .001): American Indian/Native Alaskan, 31.2% (95% CI, 24.6%-37.8%); Hispanic, 22.0% (95% CI, 19.5%-24.5%); non-Hispanic black, 20.8% (95% CI, 17.8%-23.7%); non-Hispanic white, 15.9% (95% CI, 14.3%-17.5%); and Asian, 12.8% (95% CI, 10.0%-15.6%). All pairwise differences in obesity prevalence between racial/ethnic groups were statistically significant after a Bonferroni adjustment (P < .005) except for those between Hispanic and non-Hispanic black children and between non-Hispanic white and Asian children. Conclusions Racial/ethnic disparities in obesity are apparent in 4-year-old US children. The highest prevalence is in American Indian/Native Alaskan children, in whom obesity is twice as common as in non-Hispanic white or Asian children.