BACKGROUND AND AIMS: Gastrointestinal cancer risk is influenced by the presence of metabolic syndrome [MetS]. However, previous epidemiological studies lacked full serological biomarker data for the classification of MetS and the interaction of MetS with germline cancer risk variants is unknown. METHODS: We investigated the associations between MetS and gastrointestinal cancer risk (overall, colorectal, pancreatic, esophageal adenocarcinoma, esophageal squamous cell carcinoma, stomach cardia, stomach non-cardia, hepatocellular carcinoma, and intrahepatic bile duct cancer) in 366,016 UK Biobank participants with comprehensive serum biomarker and genotype data. MetS status was determined by three different definitions at baseline and, in 15,152 participants, at a repeat assessment after a median of 4.3 years of follow-up. Multivariable hazard ratios [HR] and 95% confidence intervals [CI] for cancer outcomes were estimated using Cox proportional hazards models. Analyses stratified by polygenic risk score [PRS] were conducted for colorectal and pancreatic cancers. RESULTS: During a median follow-up of 7.1 years, 4,238 incident cases of a gastrointestinal cancer occurred. MetS at baseline was associated with higher risk of overall gastrointestinal cancer by any definition (HR 1.21, 95% CI 1.13-1.29, harmonized definition). MetS was associated with increased risks of colorectal cancer, colon cancer, rectal cancer, hepatocellular carcinoma, pancreatic cancer in women, and esophageal adenocarcinoma in men. Associations for colorectal cancer and pancreatic cancer did not differ by PRS strata (P-heterogeneity 0.70 and 0.69, respectively), and 80% of participants with MetS at baseline retained this status at the repeat assessment. CONCLUSIONS: These findings underscore the importance of maintaining good metabolic health in reducing the burden of gastrointestinal cancers, irrespective of genetic predisposition.