Mycoleptodonoides aitchisonii has been used as culinary material in East Asia. We previously reported the antiasthmatic effect of M. aitchisonii; in this study we evaluated the mushroom's immune-stimulating effects. To analyze these effects, we conducted an in vitro study with splenocytes and an in vivo study with the Porsolt enforced swim test. In the in vitro study, the cell proliferation effect of the M. aitchisonii water extract (WT) was measured and the in vivo study depended on the dose of M. aitchisonii WT and the effect on swimming duration; several immune reaction-related factors were assessed, such as white blood cell count, differential cell count, the ratio of thymus or spleen weight to body weight, and concentrations of some cytokines in serum. Almost all measured factors that are related to immune stimulation were dose-dependently increased, such as the number of splenocytes and monocytes, the swimming duration, the relative weight of the thymus, and the expression of tumor necrosis factor (TNF)-α and interferon (IFN)-γ. Based on the results, we conclude that M. aitchisonii WT has an immune-stimulating effect via TNF-α and IFN-γ, and that M. aitchisonii WT is an immune-stimulating agent and can be produced as a functional food.