The objective was to determine the effects of prebiotics and synbiotics on adults with functional constipation (FC). Medline, Embase and the Cochrane Library were searched for literature published up to February 2015. We selected randomized controlled trials (RCTs) that reported administration of prebiotics or synbiotics to adults with FC. The end points included stool frequency, stool consistency and other symptoms related to constipation. Mean differences (MD) or standard mean differences (SMD) were used for continuous outcomes and risk ratios for discontinuous outcomes using a random-effects model. The Cochrane Risk of Bias Tool was used to determine the quality of the trials. Funnel plots and Egger's test were used to analyze for publication bias. We included 5 RCTs involving 199 patients who were administered prebiotics and 8 RCTs involving 825 patients who were administered synbiotics. Prebiotics increased weekly stool frequency (MD: 1.01 bowel movements/week, 95% CI: 0.04-1.99) and improved stool consistency (SMD: -0.59, 95% CI: -1.16 to -0.02). Subgroup analysis showed specific effects for galacto-oligosaccharides on stool frequency, consistency, ease of defecation and abdominal pain. Synbiotics significantly improved stool frequency (MD: 1.15 bowel movements/week, 95% CI: 0.58-1.71), consistency (SMD: 0.63, 95% CI: 0.33-0.92) and reduced whole-gut transit time (MD: 13.52, 95% CI: -26.56 to -0.49) in patients with FC. Subgroup analysis showed specific effects for fructo-oligosaccharides and probiotic combinations on stool frequency, consistency, straining defecation and bloating. Galacto-oligosaccharides and synbiotics made up of fructo-oligosaccharides with probiotic combinations may improve stool frequency, consistency and some other symptoms related to constipation.