Bengkoang (Pachyrhizus erosus (L.) Urban) is one of the most popular edible root vegetables in Indonesia. Bengkoang contains fairly large amounts of carbohydrates and crude fiber. The purpose of this research is to evaluate the immunomodulatory effect of the bengkoang fiber extract (BFE) in vitro and in vivo. BFE was prepared by heating the powder of bengkoang fiber suspended in distilled water at 121 °C for 20 min. BFE facilitated IgM production by the human hybridoma cell line HB4C5 cells. In addition, production of IgM, IgG, and IgA by mouse primary splenocytes was facilitated by BFE in a dose-dependent manner. BFE also significantly facilitated production of both interleukin-5 and interleukin-10 by splenocytes. Immunoglobulin production by lymphocytes from the spleen, Peyer's patch, and mesenteric lymph node were significantly activated by oral administration of BFE to mice for 14 days. The serum immunoglobulin levels of IgG, IgM, and IgA were also significantly enhanced. Furthermore, cytokine production by lymphocytes from the spleen, Peyer's patch, and mesenteric lymph node were also facilitated by oral administration of BFE. These results suggest that BFE has positive effects on the immune system in vitro and in vivo.