BACKGROUND Inflammatory bowel disease (IBD) was associated with elevated risk of cardiometabolic diseases in observational studies. We aimed to evaluate the observational and genetic associations of Crohn's disease (CD) and ulcerative colitis (UC) with multiple cardiometabolic outcomes. METHODS Our phenotypic and genetic association analyses included more than 400 000 participants who were free of major cardiovascular disease and diabetes at recruitment (2006-2010) and were followed up until December 2019 based on the UK Biobank. For the Mendelian randomization (MR) analyses, 415 and 273 single nucleotide polymorphisms associated with CD and UC, respectively, were selected as genetic instruments. Summary-level data on individual cardiometabolic outcomes were obtained from 4 different genome-wide association studies with a total of 2 248 842 participants. RESULTS In the multivariable-adjusted observational analyses, CD was associated with higher risks of heart failure (hazard ratio [HR], 1.72; 95% confidence interval, 1.22-2.42) and type 2 diabetes (HR, 2.11; 95% confidence interval, 1.67-2.67) but not with myocardial infarction or ischemic stroke. UC was related to increased risks of all the assessed cardiometabolic diseases (HRs ranged from 1.29 for myocardial infarction to 1.76 for type 2 diabetes). Conversely, neither the genetic risk score for CD nor that for UC was associated with higher risk of developing cardiometabolic diseases. In 2-sample MR analyses, genetically determined CD and UC were not associated with any of the assessed cardiometabolic diseases (all P values >.05). CONCLUSIONS Despite confirming the observational associations, our study does not support a causal association between IBD and elevated risk of cardiometabolic diseases.