Background: Vitamin D status has been found to be inversely associated with risk of respiratory tract infections (RTIs). Although vitamin D status varies by ethnicity, the relationship between serum 25-hydroxyvitamin D (25[OH]D) and RTIs in United Kingdom ethnic groups remains unclear. Objectives: This study aimed to investigate the association between serum 25(OH)D status and hospitalization for RTI in United Kingdom adults. Methods: An unmatched case-control study was conducted using data from United Kingdom Biobank, which includes 500k adults with serum 25(OH)D status and hospital episodes from linked records. Survival analyses and binary logistic regression models were used to explore the association between serum 25(OH)D and RTIs. Results: Of the 36,258 participants included in the analysis, 34% were White, 28% Asian, 19% Black, 11% other, and 7% of mixed ethnicity. The RTI rate was 8.5% (median time to RTI, 14.8 y). Higher serum 25(OH)D (each +10 nmol/L increase) was significantly associated with a 4% lower hazard ratio (HR) for RTI hospitalization [HR: 0.96, 95% confidence interval (CI), 0.94, 0.99]. When stratifying for serum 25(OH)D, compared to those with ≥75 nmol/L (reference), those with <15 nmol/L had a higher HR for RTI hospitalization (HR: 1.33, 95% CI: 1.05, 1.67). Categories 15 to 24 nmol/L, 24 to 49 nmol/L, and 50 to 74 nmol/L were not statistically significant. Logistic regression models supported the above findings. Inclusion of an interaction term for 25(OH)D × ethnicity was trialed in the survival analysis, but the interaction term was not statistically significant. Conclusions: Serum 25(OH)D status <15 nmol/L is associated with 33% higher HR for RTI hospitalization among United Kingdom adults, compared with ≥75 nmol/L. Furthermore, studies are warranted to validate these findings and explore the mechanisms underlying the association between vitamin D status and RTIs in different ethnic groups.