Background and Purpose: This study aimed to determine the association between vitamin D serum levels and the severity of COVID-19 in Iran. 
Materials and Methods: This cross-sectional study was done on 121 COVID-19 patients referred to Kowsar Hospital of Semnan City, Iran, in 2022. The patients were assigned to the mild/moderate (n=80) and severe (n=41) COVID-19 groups. The data collection tool was a checklist that included demographics, clinical variables, and laboratory parameters extracted from the medical file and interviews with the patient. Also, patients’ serum vitamin D level was measured using the liquid chromatography-tandem mass spectrometry method. Data were entered in Stata software, version 14 and analyzed by multivariate logistic regression. P<0.05 were considered significant.
Results: The mean age of severe and mild/moderate groups were 67.66±16.15 and 48.5±18.7 years, respectively. About 61% and 55% of mild/moderate and severe groups were male. The multivariate logistic regression model showed vitamin D (odds ratio [OR]=0.55; 95% CI, 0.42%, 0.83%) is the most important factor predicting the severity of COVID-19 so that, for one ng/mL increase in vitamin D level, the odds of contracting the severe form of COVID-19 decreases by about 45%. 
Conclusion: The present study shows that low vitamin D levels may increase the risk of severe COVID-19. However, further studies with larger sample sizes are recommended.