Background: Vitamin D might have beneficial potential in influencing the natural history of the coronavirus disease 2019 (COVID-19) due to its immunomodulatory and anti-inflammatory properties. Objective: The aim was to investigate whether vitamin D deficiency is associated with COVID-19 incidence and disease severity in Chinese people. Methods: In a cross-sectional study we retrospectively analyzed 335 COVID-19 patients (median: 56.0; IQR: 43.0- 64.0 y) who were admitted to the Wuhan Tongji Hospital between 27 February and 21 March 2020. We also included an ageand sex-matched population of 560 individuals (median: 55; IQR: 49.0-60.0 y) who underwent the physical examination program. Their serum 25-hydroxyvitamin D [25(OH)D] concentrations were measured during the same period from 2018-2019. Serum 25(OH)D concentrations were measured for all COVID-19 patients on admission. Severity of COVID-19 was determined based on the level of respiratory involvement. A general linear model with adjustment for covariates was used to compare 25(OH)D concentrations between the COVID-19 and 2018-2019 control groups. Adjusted ORs with 95% CIs for associations between vitamin D status and COVID-19 severity were estimated via multivariable logistic regression. Results: In the general linear model adjusted for age, sex, comorbidities, and BMI, serum 25(OH)D concentrations were significantly lower among COVID-19 patients than the 2018-2019 controls [ln transformed values of 3.32 0.04 vs. 3.46 +/- 0.022 ln (nmol/L), P = 0.014]. Multivariable logistic regression showed that male sex (OR: 2.26; 95% CI: 1.06, 4.82), advanced age (>= 65 y) (OR: 4.93; 95% CI: 1.44, 16.9), and vitamin D deficiency (<30 nmol/L) (OR: 2.72; 95% CI: 1.23, 6.01) were significantly associated with COVID-19 severity (all P < 0.05). Conclusions: These findings suggested that vitamin D deficiency impacts COVID-19 hospitalization and severity in the Chinese population. J Nutr 2021;151:98-103.