BackgroundPrevious epidemiological observational studies have reported an association between inflammatory bowel disease (IBD) and prostate cancer (PCa), but the causality is inconclusive. The purpose of this study was to evaluate the causality of IBD on PCa using the mendelian randomization (MR) analysis.MethodsWe performed a two-sample MR analysis with public genome-wide association studies (GWAS) data. Eligible instrumental variables (IVs) were selected according to the three assumptions of MR analysis. The inverse-variance weighted (IVW) method was the main method. Complementary methods included the MR-Egger regression, the Weighted Median, the Simple Mode, the Weighted Mode and MR pleiotropy residual sum and outlier (MR-PRESSO) methods.ResultsGenetically determined IBD did not have a causal effect on PCa (IVWP> 0.05). Additionally, there was no causal effect of Crohn’s disease (CD) and ulcerative colitis (UC) on PCa in the MR analysis (IVWP> 0.05). Results of complementary methods were consistent with those of the IVW method.ConclusionsThis study does not support a causal association of IBD on PCa, which is in contrast to most observational studies.