Background: As an immune modulator, vitamin D has been implicated in the coronavirus disease 2019 (COVID-19) severity. This study aimed to investigate the association between vitamin D levels and the severity of COVID-19 infection. Methods: A cross-sectional study, which included 124 patients diagnosed with COVID-19 and were selected from Ain Shams University Hospitals and assigned to two groups; mild and severe COVID-19. All patients underwent detailed history taking, clinical data, and different laboratory investigations as complete blood count, blood urea nitrogen, serum creatinine, liver enzymes, C-reactive protein, D-dimer, ferritin and serum vitamin D concentration. In addition to findings of initial chest computed tomography (CT) were recorded. COVID-19 Reporting and Data System (CO-RADS) and CT chest severity scores (CT SS) were reported. Results: In this study of 124 COVID-19-positive individuals, a high prevalence of hypovitaminosis D was found (97.6%). Lower vitamin D levels were significantly associated with more severe COVID-19 cases ( p-value < 0.001), higher blood levels of inflammatory markers including (D-dimer, CRP, and ferritin), a higher CT SS and longer disease duration. Serum vitamin D can be used as a predictor for the severity of COVID-19 infection with a specificity of 96.6%, and sensitivity of 45.5%. Conclusion: The high frequency of hypovitaminosis D in severe COVID-19 patients provides further evidence of a potential link to poor prognosis and severity of the disease, so vitamin D deficiency may be a marker of poor prognosis in these patients.