OBJECTIVE We calculated the racial/ethnic-specific percentages of gestational diabetes mellitus (GDM) attributable to overweight and obesity. METHODS We analyzed 1 228 265 records of women aged 20 years or older with a live, singleton birth in California during 2007 to 2009. Using logistic regression, we estimated the magnitude of the association between prepregnancy body mass index and GDM and calculated the percentages of GDM attributable to overweight and obesity overall and by race/ethnicity. RESULTS The overall estimated GDM prevalence ranged from 5.4% among White women to 11.9% among Asian/Pacific Islander women. The adjusted percentages of GDM deliveries attributable to overweight and obesity were 17.8% among Asians/Pacific Islander, 41.2% among White, 44.2% among Hispanic, 51.2% among Black, and 57.8% among American Indian women. Select Asian subgroups, such as Vietnamese (13.0%), Asian Indian (14.0%), and Filipino (14.2%), had the highest GDM prevalence, but the lowest percentage attributable to obesity. CONCLUSIONS Elevated prepregnancy body mass index contributed to GDM in all racial/ethnic groups, which suggests that decreasing overweight and obesity among women of reproductive age could reduce GDM, associated delivery complications, and future risk of diabetes in both the mother and offspring.