Aims Exopolysaccharide fraction from Pediococcus pentosaceus KFT18 (PE‐EPS), a lactic acid bacteria isolated from Kimchi (a Korean fermented vegetable product), was preliminary characterized and its immunostimulating effects were analysed. Methods and Results In this study, we used interferon‐γ (IFN‐γ)‐primed RAW 264·7 macrophages and CD3/CD28‐stimulated splenocytes to determine the immunotimulatory activities of PE‐EPS. Upon exposure to PE‐EPS, IFN‐γ‐primed RAW 264·7 macrophages showed significant increases in the expressions of inducible nitric oxide synthase (iNOS), tumour necrosis factor‐α (TNF‐α), interleukin (IL)‐6 and IL‐1β. Molecular data using reporter gene assay and electrophoretic mobility shift assay (EMSA) revealed that PE‐EPS upregulated transcriptional activity, DNA binding and the nuclear translocation of nuclear factor‐κB (NF‐κB). Furthermore, PE‐EPS enhanced anti‐CD3/CD28‐specific proliferation and the productions of IL‐2 and IFN‐γ in primary splenocytes. In cyclophosphamide‐induced immunosuppressed mice, pretreatment with PE‐EPS (5, 15 or 45 mg kg−1 day−1, p.o.) increased thymus and spleen indices, and improved lymphocyte and neutrophil counts. Conclusion PE‐EPS stimulated the IFN‐γ‐primed macrophages and primary splenocytes to induce immune responses and improved the cyclophosphamide‐induced immunosuppression in mice. Significance and Impact of the Study The results in this study improved our understanding of immunostimulating activity of PE‐EPS and supported its potential treatment option as a natural immunostimulant.