Circadian disruption and metabolic dysfunction are linked to chronic disease, but whether circadian syndrome, which combines metabolic, sleep, and mood traits, relates to overall and site-specific cancer risk remains unclear. We prospectively studied 367,832 cancer-free participants in the UK Biobank, of whom 82,547 (22.4%) had circadian syndrome, defined by at least 4 of 7 features: abdominal obesity, high triglycerides, low levels of high-density lipoprotein cholesterol, high blood pressure, impaired glucose regulation, short sleep, and depression. Associations with overall cancer and 23 site-specific cancers were evaluated using Cox proportional-hazards and competing-risk models, with additional dose-response, biomarker, and population-burden analyses. During follow-up, 47,695 participants developed cancer. Circadian syndrome was associated with a higher incidence of overall cancer (hazard ratio [HR], 1.12; 95% confidence interval [CI], 1.10 to 1.14). The strongest positive associations were observed for endometrial cancer (HR, 1.88; 95% CI, 1.71 to 2.08), liver cancer (HR, 1.67; 95% CI, 1.47 to 1.89), and kidney cancer (HR, 1.55; 95% CI, 1.41 to 1.70). Findings remained similar after accounting for death from other causes and across sensitivity analyses, whereas inverse associations for prostate and brain cancers were exploratory. Sex-hormone-binding globulin statistically accounted for 51.0% of the endometrial cancer association, and C-reactive protein accounted for 26.4% of the overall cancer association. The highest population-attributable fractions were estimated for endometrial (13.5%) and liver cancers (13.4%). Circadian syndrome may help identify populations at increased cancer risk and inform prevention strategies integrating circadian and metabolic health.