The number of dementia patients in the world is large, and the number of dementia patients will continue to rise in the future, which will bring a heavy social and economic burden. No interventions have been found to cure dementia. Medication can delay the progression of the disease and impose an economic burden. Some non-drug therapies often require the care of the caregiver. Probiotics, prebiotics, and synbiotics may intervene in dementia through microbiota-gut-brain axis (MGBA). However, their effectiveness and safety are still obscure and deserve further investigation. The purpose of this study is to assess the effect and safety of probiotics, prebiotics, and synbiotics in treating dementia.We will summarize and meta-analyze randomized controlled trials (RCTs) of probiotics, prebiotics, and synbiotics for the treatment of dementia. RCTs comparing probiotics, prebiotics, and synbiotics with blank control, placebo or conventional therapies will be included. RCTs comparing probiotics, prebiotics, and synbiotics plus conventional therapies with conventional therapies alone will also be included. The following electronic databases will be searched: PubMed, Cochrane Library, EMBASE, CNKI, CBM, VIP, and WAN FANG DATA. The methodological quality of RCTs will be assessed using the Cochrane risk assessment tool. All trials included will be analyzed according to the criteria of the Cochrane Handbook. Review Manager 5.3, R-3.5.1 software will be used for publication bias analysis. Grading of Recommendations Assessment, Development and Evaluation (GRADE) pro-GDT web solution will be used for evidence evaluation.This review will evaluate the effects of probiotics, prebiotics, and synbiotics on cognitive function, behavioral and psychological symptoms of dementia, quality of life (QOL), functional performance in activities of daily living, and compliance with the intervention and safety in patients with dementia.This review will provide clear evidence to assess the effectiveness and safety of probiotics, prebiotics, and synbiotics for dementia.OSF registration number: DOI 10.17605/OSF.IO/2Q3AK.