Epstein-Barr virus-induced gene 3 (EBI3) encoded protein can form heterodimers with IL-27P28, and IL-12P35 to form IL-27, and IL-35. However, IL-27 stimulates, whereas IL-35 inhibits antitumor T-cell responses. IL-27 also limits the Foxp3⁺ regulatory T cell (Treg) population, whereas IL-35 has been shown to expand Tregs and foster Treg suppressive functions. It remains unclear which group of forces are dominant during antitumor T-cell responses. In this study, we evaluated the tumor growth and antitumor T-cell responses in EBI3-deficient mice that lack both IL-27 and IL-35. We found that injecting B16 melanoma cells into EBI3-deficient C57BL/6 mice, or J558 plasmacytoma cells into EBI3-deficient BALB/c mice resulted in significantly increased tumor growth relative to those implanted in wild-type control mice. Tumors from EBI3-deficient mice contained significantly decreased proportions of CD8⁺ T cells and increased proportions of CD4⁺FoxP3⁺ Treg cells as compared to those from EBI3-intact mice. Tumor-infiltrating T cells from EBI3-deficient mice were impaired in their capacity to produce IFNγ. Phenotypically, Tregs from EBI3-deficient mice were highly suppressive and produced IL-10 in the tumor microenvironment. Depletion of Tregs or inactivation of the IL-10 pathway significantly abrogated tumor growth enhancement in Ebi3^−/− mice. Finally, we showed that Ebi3^−/− mice administered a melanoma vaccine failed to mount a CD8⁺ T-cell response and the vaccine failed to confer tumor rejection in EBI3-deficient mice. Taken together, these results suggest that Ebi3^−/− mice show a phenotype of IL-27-deficiency rather than IL-35-deficiency during anti-tumor T-cell responses. Thus, our results suggest that endogenous IL-27 is critical for both spontaneous and vaccine-induced antitumor T-cell responses.