Background: COVID-19 symptoms vary from asymptomatic cases to moderate and severe illness with patients needing hospitalization and intensive care treatment. Observational studies showed a negative association of low vitamin D levels and COVID-19 severity and mortality outcomes. Vitamin D supplementation in respiratory diseases has been shown to be beneficial, but the evidence for vitamin D supplementation in COVID-19 patients is conflicting. Aims of the study: The aims of this study were to determine the impact of vitamin D supplementation on clinical outcomes in patients with severe form of COVID-19 and vitamin D deficiency. The main aim was to determine the difference in the number of days spent on mechanical ventilation support in patients with supplementation and patients without vitamin D supplementation. Methods: Randomized clinical trial was conducted in Split Clinical Hospital Center in the period from November 2021. to May 2022. COVID-19 patients who required invasive or non-invasive respiratory support based on low vitamin D levels at admission (< 50 nmol/L) were randomized into two groups: the intervention group receiving vitamin D supplementation and the control group. A total of 155 patients were randomized: 78 to the intervention group and 77 to the control group. Patients who were randomized to the intervention group received 10.000 IU of cholecalciferol per day for 14 days. Results: In the primary outcome analysis patients in the intervention group had a median duration of mechanical ventilation of 10 days (IQR 6-13.9), and patients in the control group 8 days (IQR 6- 12) (p= 0.283). In the comparison of the survival of patients from these two groups, there was also no significant difference. Survival after 14, 28 and 60 days in the group of patients with normal levels of vitamin D was 90%, 50% and 46%, while survival in the mentioned periods in patients with low levels of vitamin D was 83%, 64.9% and 50.7% respectively. In the analysis of outcomes related to the severity of the disease and inflammatory markers, although there was a trend difference in the values of CRP, procalcitonin and the ratio of lymphocytes and neutrophils, this difference was not statistically significant. Conclusion: Vitamin D supplementation in patients with vitamin D deficiency and a severe form of COVID-19 who required invasive or non-invasive respiratory support treated in the intensive care unit during the study period did not reduce the number of days on respiratory support compared to patients who did not receive vitamin D supplementation.