The purpose of the study was to assess the effects and the protective mechanism of konjac oligosaccharide (KOS) on the ulcerative colitis (UC) model induced by 2,4,6-trinitrobenzenesulfonic acid (TNBS) in rats. KOS (1.0 and 4.0 g/kg/day) was administered for 14 days after the induction of colitis with TNBS. The status of the rats was assessed by morphological and biochemical methods. The effect of KOS on the colonic microflora was also assessed by studying the bacteria profile and short chain fatty acids (SCFAs) production in feces by standard culture techniques and gas chromatography, respectively. KOS administration improved rat weight, colonic length, damage score, structure of gut microbiota, production of SCFA, and reduced colon tissue levels of malondialdehyde (MDA), inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β). Therefore, our results indicate that KOS is an anti-inflammatory and could be useful as a prebiotic to design functional foods for UC.