ABSTRACT Aims/Introduction Diabetes is associated with poor clinical outcomes of coronavirus disease 2019 (COVID‐19). However, the impact of newly diagnosed diabetes on prognosis has not been clarified. The objective of this study was to show the features and outcome of COVID‐19 patients with newly diagnosed diabetes in Japan. Materials and Methods We retrospectively analyzed 62 patients with diabetes hospitalized for COVID‐19 between 1 April and 18 August 2021 at the National Center for Global Health and Medicine in Tokyo, Japan. We evaluated the worst severity of COVID‐19 and plasma blood glucose levels in patients with newly diagnosed diabetes or pre‐existing diabetes. Results This study included 62 confirmed COVID‐19 patients with diabetes, including 19 (30.6%) patients with newly diagnosed diabetes and 43 (69.4%) patients with pre‐existing diabetes. Patients with newly diagnosed diabetes significantly progressed to a critical condition more frequently during hospitalization than patients with pre‐existing diabetes (52.6% vs 20.9%, P = 0.018). In addition, patients with newly diagnosed diabetes had significantly higher average plasma blood glucose levels for the first 3 days after admission than those with pre‐existing diabetes. Conclusions Our study suggests that the proportion of COVID‐19 patients who are newly diagnosed with diabetes is high, and they have an increased risk of developing severe disease than those with pre‐existing diabetes. It might be advisable that at the point of COVID‐19 diagnosis, blood glucose and glycated hemoglobin levels be assessed in all patients. We retrospectively analyzed 62 patients with diabetes hospitalized for coronavirus disease 2019 at the National Center for Global Health and Medicine in Japan. Our study showed that patients with newly diagnosed diabetes had a higher severity of coronavirus disease 2019 and had difficulty in managing glycemic control compared with patients with pre‐existing diabetes.