BACKGROUND: Undernutrition predisposes children to a greater incidence and duration of diarrhea. No review and meta-analysis have yet been conducted to assess effectiveness of probiotics and synbiotics in undernourished children. AIMS: To assess the effectiveness of probiotics and synbiotics on diarrhea in undernourished children. METHODS: Randomized, double-blind, placebo-controlled trials evaluating the effects of probiotics and synbiotics on diarrhea in undernourished children were searched from 1990 to May 2020. Recommendations of the Cochrane Handbook and the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) statement were followed. RESULTS: The systematic review identified 15 trials with 6986 patients. The meta-analysis revealed that treatment with probiotic or synbiotic reduced significantly both the duration of diarrhea [Weighted mean difference (WMD) = -1.05 day, 95% CI (-1.98, -0.11)] and the hospital stay duration [Standard mean difference (SMD) = -2.87 days, 95% CI (-5.33, -0.42)], especially in specific patient subsets. In both groups, similar rates of vomiting and nutritional recovery were observed. No probiotics or synbiotics-related adverse effects were reported. Subgroup analyses showed that probiotic and synbiotic treatment were more effective in reducing risk of diarrhea in outpatients [Risk ratio (RR) = 0.86, 95%CI (0.75-0.98)]. CONCLUSION: This meta-analysis supports the potential beneficial roles of probiotics and synbiotics on diarrhea in undernourished children.