BACKGROUND A unified view of whether asthma and periodontitis interact and the direction of action is not found in previous studies. Therefore, in this article, we will elucidate bidirectional causality by two-sample Mendelian randomization (MR). METHODS We obtained summary-level data for asthma and periodontitis from the massive GWAS databases of several publicly available. Meanwhile, it will ensure no confounders like smoking using the Phenoscanner website to have a search over each SNP. The F statistic value of each SNP is calculated as more significant than 10. This MR analysis mainly used five MR methods: MR-Egger, weighted median, inverse variance weighted (IVW), simple mode, and weighted mode. As a result, we performed heterogeneity and horizontal pleiotropy tests. RESULTS We have found supporting evidence to verify the hypothesis that asthma may be a protective factor for periodontitis (IVW OR = 0.34; 95% CI = [0.132,0.87]; p = 0.025). The consistent impact direction is shown when additional asthma GWAS dataset are used (MR-Egger OR = 0.118; 95%CI = [0.016,0.883]; p = 0.04). There is no evidence of a causal effect of periodontitis at the risk of asthma in the reverse analysis (p > 0.05). CONCLUSIONS Our analysis found that people with asthma may have a lower risk of periodontitis than those without. This MR analysis could have significant implications for the clinical process and future research.