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.