AIM: This Mendelian randomization (MR) study was performed to explore the potential bidirectional causal association between inflammatory bowel disease (IBD) and periodontitis. MATERIALS AND METHODS: We used genetic instruments from the genome-wide association study summary statistics of European descent for IBD (12,882 cases and 21,770 controls) to investigate the association with periodontitis (3046 cases and 195,395 controls) and vice versa. The radial inverse-variance weighted method was carried out to obtain the primary causal estimates, and the robustness of the results was assessed by a series of sensitivity analyses. Due to multiple testing, associations with p values <.008 were considered as statistically significant, and p values ≥.008 and <.05 were considered as suggestively significant. RESULTS: In the primary causal estimates, IBD as a whole was associated with an increased risk of periodontitis (odds ratio [OR], 1.060; 95% confidence interval [CI], 1.017; 1.105; p = .006). Subtype analyses showed that ulcerative colitis (UC) was associated with periodontitis (OR, 1.074; 95% CI 1.029; 1.122; p = .001), while Crohn's disease (CD) was not. Regarding the reverse direction, periodontitis showed a suggestive association with IBD as a whole (OR, 1.065; 95% CI 1.013; 1.119; p = .014). Subtype analyses revealed that periodontitis was associated with CD (OR, 1.100; 95% CI 1.038; 1.167; p = .001) but not UC. The final models after outlier removal showed no obvious pleiotropy, indicating that our primary analysis results were reliable. CONCLUSIONS: The present MR study provides moderate evidence on the bidirectional causal relationship between IBD and periodontitis. The bidirectional increased risk found in our study was marginal and, possibly, of limited clinical relevance. More studies are needed to support the findings of our current study.