Rationale The chronic nature of diabetes renders it a risk factor for lower respiratory tract infections, adversely impacting patient prognosis. Objective This study aimed to investigate the influence of diabetes on the outcomes of coronavirus disease 2019 (COVID-19) patients. Patients and methods This prospective cohort study enrolled 160 patients admitted and diagnosed with COVID-19. Patients were categorized according to glycated hemoglobin level into two groups: diabetic and nondiabetics (80 patients each). The COVID-19 severity index was calculated for all patients. Follow-up of their in-hospital stay and incidence of complications was documented. Results It was found that diabetic patients had a significantly higher percentage of severe COVID-19 cases (83.8 vs. 61.3%) [odds ratio (OR)=1.89, 95% confidence interval (CI)=1.02–3.48]. The diabetic group had higher rates of ICU admission (OR=2.21, 95% CI=1.06–4.57), invasive mechanical ventilation (OR=3.75, 95% CI=1.321–10.803), and mortality (HR=1.69, 95% CI=1.132-2.514) compared with nondiabetic patients (33.8 vs. 18.8%, 20 vs. 6%, and 41.5 vs. 21.2%, respectively). From the nondiabetic group, 68.4% of prediabetic patients had hyperglycemia requiring insulin treatment during their hospital stay versus 31.6% of normoglycemic patients. Conclusion Diabetes mellitus is associated with more severe manifestations of the disease, increased complications, ICU admissions, ventilatory requirements, and mortality rates compared to nondiabetic individuals. COVID-19 is associated with poor glycemic control.