© 2021 Merve Ergin Tuncay et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License.Background: Vitamin D is recognized to be an immune regulator. Also, it is known to have antiviral effects by several mechanisms, including reducing inflammatory cytokines. Objectives: To examine the 25-hydroxyvitamin D (25(OH) D) status for assessing the severity of COVID-19. Methods: This study consisted of 596 patients confirmed as SARS-CoV-2 infection and 59 healthy individuals. The cases separated into non-severe group, severe survival, and severe non-survival group. 25(OH)D and other laboratory parameters were evaluated retrospectively. Results: In all COVID-19 groups 25(OH)D levels were low compared to controls (p<0.05). 25(OH)D concentrations were lowest in patients in severe non-survival groups than those in other SARS-CoV-2 infection groups (p<0.05). Multivariate regression analysis exhibited that decreasing 25(OH)D was associated with an increased likelihood of non-severe, severe survival and severe non-survival disease. There were significant associations between 25(OH)D and certain inflammatory and hemostatic parameters (p<0.05, for all). Conclusions: 25(OH)D deficiency was observed among patients with COVID-19. Declined steadily 25(OH)D levels make a huge contribution to the scale of the progression of the disease. Correlations support that 25(OH)D may be a substantial tool for utilizing the severity of the disease and estimating the survival. Also, supplementation of 25(OH)D might slow down the course of the COVID-19.