Immunity is the defense against external invasion and breakdown of the immune system that causes various diseases. In this study, the immunomodulatory function of brown rice fermented vinegar (FV) was investigated using a cyclophosphamide (CP)-induced immunosuppression model. In the in vitro study, splenocytes were treated with CP to induce immunosuppression and to measure splenocyte proliferation and natural killer (NK) cell activity. Here, 5 mg/kg of CP was orally administered to induce immunosuppression in the animal model, and was then orally treated with 0.5 ml/kg, 1 ml/kg, and 2 ml/kg of FV each. The results from the in vitro studies showed that CP treatment decreased the activities of splenocytes, cytokines, and NK cells, which was later increased with FV treatment. Additionally, in the CP-induced immunosuppression animal model the number of immune cells, cytokines, and weights of the immune organs decreased, and the spleen tissues were destroyed compared to the normal control group. However, in the FV-treated groups, the immune cells and cytokines increased, in addition to the weights of the liver, thymus, and spleen, and the damaged spleen tissues recovered. Therefore, these results demonstrate that FV had an immunomodulatory function.