Background <p>The link between vitamin D and cancer remains inconclusive. In this study, we aimed to explore the relationship between circulating vitamin D levels and overall and site-specific cancers in Korean adults using data from two large prospective cohort studies.</p> Methods <p>Baseline serum 25-hydroxyvitamin D [25(OH)D] levels were measured in a subset of participants from the Cardiovascular Disease Association Study (2005–2012) and the Health Examinees Study (2009–2013). We followed 46,514 adults aged ≥ 40 years who consented to linkage with national cancer registry data. Cox proportional hazards regression was used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for cancer incidence according to quartiles of season-standardized 25(OH)D levels.</p> Results <p>The median season-standardized 25(OH)D level was 45.6 nmol/L (interquartile range: 33.6–59.7 nmol/L). During the median follow-up of 10.6 years, 3,529 incident cancer cases were recorded. Compared with the first quartile, the upper quartiles of serum 25(OH)D were associated with a lower risk of overall cancer [HR (95% CI): 0.86 (0.77–0.95), 0.84 (0.75–0.93), and 0.80 (0.72–0.89), respectively; <i>P</i> for trend &lt; 0.001]. For site-specific cancers, the HRs (95% CIs) for the comparison of extreme quartiles of serum 25(OH)D were 0.72 (0.52–0.99) for colorectal cancer, 0.32 (0.21–0.50) for liver cancer, and 0.75 (0.55–1.04) for lung cancer. Upon categorization of serum 25(OH)D levels based on absolute cut-off points, participants with levels ≥ 75 nmol/L had significantly lower risks of overall, liver, and lung cancers compared with those with levels &lt; 30 nmol/L.</p> Conclusions <p>These findings suggest that higher 25(OH)D levels are associated with a lower risk of overall and some site-specific cancers in the Korean population.</p> Clinical trial number <p>Not applicable.</p>