Probiotics are well-known to be directly or indirectly involved in the host immune system. In this study, we analyzed the immune-boosting effects of lactic acid bacteria, including <i>Limosilactobacillus</i> and <i>Lactococcus</i>, in immunocompetent C57BL/6J mice. Three different lactic acid bacteria strains were orally administered to C57BL/6J mice for 8 weeks. Then, liver, spleen, and whole blood were harvested after sacrificing the animals. There were no significant changes in whole-body weight, weight of organs, or complete blood cell count by oral administration of lactic acid bacteria. The frequencies of CD3<sup>+</sup>, CD4<sup>+</sup>, and CD8<sup>+</sup> T cells were significantly increased in the <i>Limosilactobacillus reuteri</i> MG5462 group compared to control. The frequency of NK1.1<sup>+</sup> cells was significantly increased in the <i>Lactococcus lactis</i> MG5474 group compared to control. On the other hand, splenocyte proliferations and natural killer cytotoxicity did not differ between groups. In addition, the MG5462 group had a significant increase in the production of TNF-α compared to the control, which is consistent with the upregulation of T cells in the MG5462 group. Therefore, <i>Limosilactobacillus reuteri</i> could be a functional food additive to boost immunity by positively affecting T cell populations.