CONTEXT: Several studies have suggested an association between functional defecation disorders (FDDs) and overweight/obesity in children. OBJECTIVE: To synthesize current evidence evaluating the association between FDDs and overweight/obesity in children. DATA SOURCES: PubMed, Medline, and Embase were searched from inception until January 25, 2016. STUDY SELECTION: Prospective and cross-sectional studies investigating the association between FDDs and overweight/obesity in children 0 to 18 years were included. DATA EXTRACTION: Data generation was performed independently by 2 authors and quality was assessed by using quality assessment tools from the National Heart, Lung, and Blood Institute. RESULTS: Eight studies were included: 2 studies evaluating the prevalence of FDDs in obese children, 3 studies evaluating the prevalence of overweight/obesity in children with FDDs, and 3 population-based studies. Both studies in obesity clinics revealed a higher prevalence of functional constipation (21%-23%) compared with the general population (3%-16%). In 3 case-control studies, the prevalence of overweight (12%-33%) and obesity (17%-20%) was found to be higher in FDD patients compared with controls (13%-23% and 0%-12%, respectively), this difference was significant in 2/3 studies. One of 3 population-based studies revealed evidence for an association between FDDs and overweight/obesity. Quality of 7/8 studies was rated fair or poor. LIMITATIONS: Due to heterogeneity of the study designs, we refrained from statistically pooling. CONCLUSIONS: Although several studies have revealed the potential association between FDDs and excessive bodyweight in children, results across included studies in this review differ strongly and are conflicting. Therefore, this systematic review could not confirm or refute this association.