Introduction. This study aims to describe the trends and racial/ethnic differences in prevalence of severe obesity among US children. Methods. We examined height and weight measurements from US children and adolescents aged 2 to 19 years using three National Health and Nutrition Examination Surveys (NHANES II: 1976–1980, N = 7 201, NHANES III: 1988–1994, N = 10 600, and NHANES 1999–2006, N = 15 980). We estimated the prevalence of severe obesity, defined as having a body mass index (BMI) above 35 kg/m2 or 120% of the age- and sex-specific 95th percentile on the CDC reference standards, stratified by sex and age groups (aged 2–5 years, 6–11 years, and 12–19 years). We compared prevalence estimates among non-Hispanic white, non-Hispanic black, and Hispanic children. Results. The age-standardized prevalence of severe obesity increased from 1.1%/1.3% (boys/girls) in NHANES II, to 2.9%/3.1% in NHANES III and 5.1%/4.7% in NHANES 1999–2006 (p-values for trend <0.001). We observed some differences in prevalence by race/ethnicity. Hispanic boys 6–11 years of age (9.0%, 95% CI: [6.4, 11.5]) and non-Hispanic black girls 12–19 years of age (12.6%, 95% CI: [10.6, 14.6]) had the highest prevalence of severe obesity in the most recent NHANES data. On average, obese youth were 9.7 kg (95% CI: [9.2, 10.2]) above the obesity standard, and severely obese youth were 21.3 kg (95% CI: [20.5, 22.3]) above. Conclusions. The prevalence of severe obesity among US youth is increasing, especially among Hispanic boys and non-Hispanic black girls. Our findings suggest that the degree of pediatric obesity is substantial and will likely have profound impact on adult morbid obesity and other morbidities.