Tetragenococcus halophilus is a halophilic lactic acid bacterium that exists in the traditional Japanese seasoning miso-a fermented soy paste. Considering the popularity of miso as a component of healthy diet, we attempted to evaluate the immunoregulatory functions of T. halophilus spices isolated from miso. We screened 56 strains that facilitated the upregulation of activation markers such as CD86 and CD69 on B cells and T cells in vitro. Of these, 7 strains (Nos. 1, 3, 13, 15, 19, 30, and 31) were found to preferentially induce the CD86 expression on B cells. Furthermore, DNA microarray analysis revealed that T. halophilus strain No. 1 significantly augmented the gene expressions of CD86, CD70, IL-10, INF-γ, and IL-22 in B cells. We confirmed these results at the protein level by flow cytometry. Mice feeding diet containing 1% T. halophilus No. 1 exhibited significantly greater IgA production in the serum. Furthermore, a diet containing 1% T. halophilus No. 1 augmented ovoalbumin (OVA)-specific IgG titer in mice upon OVA/alum immunization. Thus, we demonstrated that T. halophilus No. 1 is a strong immunomodulatory strain with potential as a probiotic.