Background and aims: We describe mortality-related risk factors of inpatients with diabetes and coronavirus disease 2019 (COVID-19) in Belgium. Methods: We conducted a multicenter retrospective study from March to May, 2020, in 8 Belgian centers. Data on admission of patients with diabetes and COVID-19 were collected. Survivors were compared to non-survivors to identify prognostic risk factors for in-hospital death using multivariate analysis in both the total population and in the subgroup of patients admitted in the intensive care unit (ICU). Results: The study included 375 patients. The mortality rate was 26.4% (99/375) in the total population and 40% (27/67) in the ICU. Multivariate analysis identified older age (HR 1.05 [CI 1.03–1.07], P < 0.0001) and male gender (HR 2.01 [1.31–3.07], P = 0.0013) as main independent risk factors for in-hospital death in the total population. Metformin (HR 0.51 [0.34–0.78], P = 0.0018) and renin-angiotensin-aldosterone system blockers (HR 0.56 [0.36–0.86], P = 0.0088) use before admission were independent protective factors. In the ICU, chronic kidney disease (CKD) was identified as an independent risk factor for death (HR 4.96 [2.14–11.5], P < 0.001). Conclusion: In-hospital mortality due to the first wave of COVID-19 pandemic in Belgium was high in patients with diabetes. We found that advanced age and male gender were independent risk factors for in-hospital death. We also showed that metformin use before admission was associated with a significant reduction of COVID-19-related in-hospital mortality. Finally, we showed that CKD is a COVID-19-related mortality risk factor in patients with diabetes admitted in the ICU. ; SCOPUS: ar.j ; info:eu-repo/semantics/published