Breast cancer is a major cause of cancer-related death in women. Antitumor T cell responses play critical therapeutic roles, including direct cytotoxicity mediated by CD8⁺ T cells and immunomodulatory roles mediated by CD4⁺ T cells. The IL-9-expressing Th9 cells are recently found to present antitumor immunity in melanoma and lung adenocarcinoma. In this study, we found that IL-9 expression in the serum and in circulating CD4⁺ T cells were significantly upregulated in breast cancer patients compared to healthy controls. The IL-9-expressing Th9 cells were enriched in the CCR4⁻ CCR6⁻ CXCR3⁻ subset. Upon TCR stimulation, this subset also presented potent IL-10 and IL-21 expression in addition to IL-9 expression. CCR4⁻ CCR6⁻ CXCR3⁻ CD4⁺ T cells also assisted in the killing of autologous tumor cells by CD8⁺ T cells, but did not initiate cytotoxicity by themselves. This enhancement in CD8⁺ T cell-mediated cytotoxicity was dependent on IL-9 as well as on IL-21. Interestingly, the tumor-infiltrating Th9 cells presented comparable IL-9, reduced IL-10, and elevated IL-21 expression compared with their counterparts in the peripheral blood. Together, these results demonstrated that IL-9-expressing Th9 cells were upregulated in breast cancer patients and potentially possessed antitumor roles by enhancing CD8⁺ T cell-mediated cytotoxicity.