Poor access to care and lack of health insurance are important contributors to disparities in glycemic control. However expanding health insurance coverage may not be enough to fully address the high burden of poor glycemic control for some groups. To characterize racial and ethnic disparities in glycemic control among adults with private and public insurance in the US over a 15-year timeframe and to evaluate whether social, health care, and behavioral or health status factors attenuate estimates of disparities. This cross-sectional study used data from the National Health and Nutrition Examination Survey from 2003 to 2018. Participants included Hispanic or Latino, non-Hispanic Black, and non-Hispanic White adults aged 25 to 80 years with self-reported diabetes and health insurance. Data were analyzed from January 15 to August 23, 2023. Participants self-identified as Hispanic or Latino, non-Hispanic Black, or non-Hispanic White. The main outcome, poor glycemic control, was defined as glycated hemoglobin A1c (HbA1c) of 7.0% or greater. Information about social (education, food security, and nativity), health care (insurance type, routine place for health care, insurance gap in past year, and use of diabetes medications), and behavioral or health status (years with diabetes, waist circumference, and smoking) factors were collected via questionnaires. A total of 4070 individuals (weighted mean [SE] age, 61.4 [0.27] years; 1970 [weighted proportion, 49.3%] were women) were included, representing 16 337 362 US adults, including 1146 Hispanic or Latino individuals (weighted proportion, 13.2%), 1196 non-Hispanic Black individuals (weighted proportion, 15.7%), and 1728 non-Hispanic White individuals (weighted proportion, 71.1%). In models adjusted for age, sex, and survey year, Hispanic or Latino and non-Hispanic Black individuals had significantly higher odds of poor glycemic control than non-Hispanic White individuals (Hispanic or Latino: odds ratio [OR], 1.46; 95% CI, 1.16-1.83; Black: OR, 1.28; 95% CI, 1.04-1.57). There was some attenuation after adjustment for social factors, especially food security (Hispanic or Latino: OR, 1.39; 95% CI, 1.08-1.81); Black: OR, 1.39; 95% CI, 1.08-1.81). However, accounting for health care and behavioral or health status factors increased disparities, especially for Hispanic or Latino individuals (OR, 1.63; 95% CI, 1.24-2.16), with racial and ethnic disparities persisting even among those with private insurance (OR, 1.66; 95% CI, 1.10-2.52). In this cross-sectional study of insured adults with diabetes in the US, disparities in poor glycemic control persisted despite adjustment for social, health care, and behavioral factors. Research is needed to identify the barriers contributing to poor control even in populations with access to care.