BACKGROUND Vitamin D plays a role in immune regulation and may influence COVID-19 outcomes. This study examined the association between serum vitamin D levels with COVID-19 infection and mortality in symptomatic patients in Taiwan. METHODS This retrospective cohort study included symptomatic patients who underwent COVID-19 real-time polymerase chain reaction (RT-PCR) assay between 2020 and 2023. Serum 25-hydroxyvitamin D (25(OH)D) levels were categorized as deficient (<20 ng/mL), insufficient (20-30 ng/mL), or sufficient (≥30 ng/mL). Logistic regression models were used to evaluate the impact of vitamin D levels on COVID-19 infection and mortality, adjusting for potential confounders including age, sex, and comorbidities. RESULTS Among the 481 participants (mean age 58.2 years; 66.7% female), 96 (19.96%) tested positive for COVID-19. Vitamin D level was not associated with COVID-19 infection (OR=1.00, 95% CI: 0.99-1.02, p=0.687), however vitamin D deficiency significantly increased the risk of COVID-19-related mortality (OR=10.68, 95% CI: 1.18-96.45; p=0.035). The mortality rate in patients aged 65-74 years was higher compared to those aged 19-44 years (OR=12.91, 95% CI: 1.29-128.83; p=0.029). CONCLUSIONS Vitamin D levels were not associated with susceptibility to COVID-19 infection. However, vitamin D deficiency, particularly in older patients, was associated with an increased risk of mortality among those diagnosed with COVID-19. These findings support the role of vitamin D in reducing COVID-19 mortality, and emphasize the importance of maintaining sufficient levels in high-risk populations.