Myeloid-derived suppressor cells (MDSCs) contribute to tumor-mediated immune escape by suppressing antitumor immune responses. Interleukin-33 (IL-33) is capable of regulating various immune cell populations; however, the effects of IL-33 on the differentiation of MDSCs have not been well characterized. In this study, we evaluated the effects of IL-33 on MDSCs and found that IL-33 significantly reduced the differentiation of lineage-negative bone marrow progenitor cells into granulocytic MDSCs (G-MDSCs). IL-33-treated MDSCs exhibited diminished immunosuppressive capacity; reduced inhibition on T-cell proliferation and interferon-γ production, and diminished production of reactive oxygen species. However, IL-33 treatment did not affect the frequency of monocytic MDSCs (M-MDSCs) or their production of nitric oxide and expression of arginase-1. Additionally, compared with control MDSCs, IL-33-treated MDSCs had reduced capacity to induce the differentiation or expansion of Treg cells. Moreover, in vivo IL-33 administration significantly decreased MDSCs and G-MDSCs accumulation in the spleen and tumor microenvironment. Also, despite increasing CD4+ and CD8+ T-cell infiltration, IL-33 administration markedly decreased Treg-cell population in tumor microenvironment. Taken together, our findings indicate that IL-33 reduces the frequency and immunosuppressive activity of G-MDSCs and ultimately the extent of tumor growth.