To examine racial/ethnic disparities in the prevalence of diabetes and prediabetes by BMI category. In a consortium of three U.S. integrated health care systems, 4,906,238 individuals aged ≥20 years during 2012-2013 were included. Diabetes and prediabetes were ascertained by diagnosis and laboratory results; antihyperglycemic medications were also included for diabetes ascertainment. The age-standardized diabetes and prediabetes prevalence estimates were 15.9% and 33.4%, respectively. Diabetes but not prediabetes prevalence increased across BMI categories among all racial/ethnic groups (P for trend < 0.001). Racial/ethnic minorities reached a given diabetes prevalence at lower BMIs than whites; Hawaiians/Pacific Islanders and Asians had a diabetes prevalence of 24.6% (95% CI 24.1-25.2%) in overweight and 26.5% (26.3-26.8%) in obese class 1, whereas whites had a prevalence of 23.7% (23.5-23.8%) in obese class 2. The age-standardized prediabetes prevalence estimates in overweight among Hispanics (35.6% [35.4-35.7%]), Asians (38.1% [38.0-38.3%]), and Hawaiians/Pacific Islanders (37.5% [36.9-38.2%]) were similar to those in obese class 4 among whites (35.3% [34.5-36.0%]), blacks (36.8% [35.5-38.2%]), and American Indians/Alaskan Natives (34.2% [29.6-38.8%]). In adjusted models, the strength of association between BMI and diabetes was highest among whites (relative risk comparing obese class 4 with normal weight 7.64 [95% CI 7.50-7.79]) and lowest among blacks (3.16 [3.05-3.27]). The association between BMI and prediabetes was less pronounced. Racial/ethnic minorities had a higher burden of diabetes and prediabetes at lower BMIs than whites, suggesting the role of factors other than obesity in racial/ethnic disparities in diabetes and prediabetes risk and highlighting the need for tailored screening and prevention strategies. © 2019 by the American Diabetes Association.