Data sources The electronic databases Medline, the Cochrane Central Register of Controlled Trials, Embase, China National Knowledge Infrastructure, China Science and Technology Journal Database, Wanfang Data, ClinicalTrials.gov and WHO International Clinical Trials Registry Platform, from inception to September 2020, were searched to identify the eligible studies measuring the association between periodontal disease and Alzheimer's disease or mild cognitive impairment.Study selection Cohort, cross-sectional and case-control studies, without any language restrictions, were selected by two reviewers independently.Data extraction and synthesis Data extraction and quality assessment were performed by two reviewers independently. Data was synthesised quantitatively with meta-analyses using a random or fixed-effects model, with P0.1 considered statistically significant. Quality assessment of cohort and case-control studies was carried out using the Newcastle-Ottawa scale (NOS) and quality assessment of cross-sectional studies was undertaken using the Agency for Healthcare Research and Quality (AHRQ) tool. Heterogeneity of included studies was assessed with I2.Results Thirteen studies, including five cross-sectional studies, five case-control studies, two retrospective cohort studies and one prospective cohort study were found to be eligible. Meta-analyses showed elevated risk for Alzheimer's disease (odds ratio = 1.78; random-effects model; significant heterogeneity) and mild cognitive impairment (odds ratio = 1.60; fixed-effects model; low heterogeneity) in patients with periodontal disease. One case-control study and all cohort studies had high quality, while four case-control studies had medium quality, as evaluated by the NOS. Among the cross-sectional studies evaluated by the AHRQ tool, only one had high quality, whereas other studies had medium quality.Conclusions Within the limitations of the included studies, the authors concluded that periodontal disease is related to an elevated risk of Alzheimer's disease and mild cognitive impairment.