Aims/Introduction To investigate overlooked diabetes in patients with coronavirus disease 2019 (COVID‐19). Materials and Methods In total, 462 COVID‐19 inpatients were included in this retrospective study. The presence of diabetes before COVID‐19 admission, and the HbA1c and blood glucose levels at admission were examined. Results Of the 462 patients, 116 had diabetes. Seventy‐six patients had been diagnosed with diabetes before COVID‐19 admission, and 40 patients were diagnosed for the first time. Of the patients with diabetes 72% required insulin. Patients with diabetes were significantly ( P < 0.05) older, more likely to be male, heavier, and showed a lower eGFR. Patients with overlooked diabetes showed a lower HbA1c (average 7.1% vs 7.5%), a lower casual blood glucose (average 157 vs 179 mg/dL), and they used less insulin per day during hospitalization (average 16.0 units vs 34.5 units) than patients with previously diagnosed diabetes. Patients with overlooked diabetes tended to have more severe COVID‐19 than those with pre‐diagnosed diabetes. Multivariable logistic regression analyses showed that the increased odds ratios (ORs) of aggravation in all patients with COVID‐19 were associated with age [OR 1.04], BMI [OR 1.05], and diabetes [OR 2.15]. The risk factors for aggravation in patients with COVID‐19 and diabetes were age [OR 1.05] and HbA1c [OR 1.45]. Conclusions Diabetes is a predictor of COVID‐19 aggravation. Furthermore, in COVID‐19 patients with diabetes, high HbA1c levels are a risk factor for severe COVID‐19. A total of 8.7% of COVID‐19 inpatients were diagnosed with diabetes after HbA1c was measured on admission. Therefore, it is important to measure HbA1c in COVID‐19 patients.