OBJECTIVE To examine the associations of circulating 25-hydroxyvitamin D (25[OH]D) concentrations with cardiovascular disease (CVD) and all-cause mortality in individuals with prediabetes and diabetes from the large population-based UK Biobank cohort study. RESEARCH DESIGN AND METHODS A total of 67,789 individuals diagnosed with prediabetes and 24,311 with diabetes who had no CVD or cancer at baseline were included in the current study. Serum 25(OH)D concentrations were measured at baseline. Cox proportional hazard models were used to calculate hazard ratios (HRs) and 95% CIs for cardiovascular outcomes and mortality after 10–14 years. RESULTS After multivariable adjustment, higher serum 25(OH)D levels were significantly and nonlinearly associated with lower risk of cardiovascular outcomes and all-cause mortality among participants with prediabetes and diabetes (all P nonlinearity < 0.05). Compared with those in the lowest category of 25(OH)D levels (<25 nmol/L), participants with prediabetes in the highest category of 25(OH)D levels (≥75 nmol/L) had a significant association with lower risk of cardiovascular events (HR 0.78; 95% CI 0.71–0.86), coronary heart disease (CHD) (HR 0.79; 95% CI 0.71–0.89), heart failure (HR 0.66; 95% CI 0.54–0.81), stroke (HR 0.75; 95% CI 0.61–0.93), CVD mortality (HR 0.43; 95% CI 0.32–0.59), and all-cause mortality (HR 0.66; 95% CI 0.58–0.75). Likewise, these associations with cardiovascular events, CHD, heart failure, CVD mortality, and all-cause mortality were observed among participants with diabetes, except for stroke. CONCLUSIONS These findings highlight the importance of monitoring and correcting vitamin D deficiency in the prevention of CVD and mortality among adults with prediabetes and diabetes.