The beneficial effects of carbohydrate-derived fibers are mainly attributed to modulation of the microbiota, increased colonic fermentation, and the production of short-chain fatty acids. We studied the direct immune responses to alimentary fibers in in vitro and in vivo models. Firstly, we evaluated the immunomodulation induced by nine different types of low-digestible fibers on human peripheral blood mononuclear cells. None of the fibers tested induced cytokine production in baseline conditions. However, only one from all fibers almost completely inhibited the production of anti- and proinflammatory cytokines induced by bacteria. Secondly, the impact of short- (five days) and long-term (three weeks) oral treatments with selected fibers was assessed in the trinitrobenzene-sulfonic acid colitis model in mice. The immunosuppressive fiber significantly reduced levels of inflammatory markers over both treatment periods, whereas a nonimmunomodulatory fiber had no effect. The two fibers did not differ in terms of the observed fermentation products and colonic microbiota after three weeks of treatment, suggesting that the anti-inflammatory action was not related to prebiotic properties. Hence, we observed a direct effect of a specific fiber on the murine immune system. This intrinsic, fiber-dependent immunomodulatory potential may extend prebiotic-mediated protection in inflammatory bowel disease.