IFN-gamma plays a central role in antitumor immunity. T cell Ig and mucin domain (Tim-3) is expressed on IFN-gamma-producing Th1 cells; on interaction with its ligand, galectin-9, Th1 immunity is terminated. In this study, we show that transgenic overexpression of Tim-3 on T cells results in an increase in CD11b(+)Ly-6G(+) cells and inhibition of immune responses. Molecular characterization of CD11b(+)Ly-6G(+) cells reveals a phenotype consistent with granulocytic myeloid-derived suppressor cells. Accordingly, we find that modulation of the Tim-3/galectin-9 (Gal-9) pathway impacts on tumor growth. Similarly, overexpression of Tim-3 ligand, Gal-9, results in an increase in CD11b(+)Ly-6G(+) cells and inhibition of immune responses. Loss of Tim-3 restores normal levels of CD11b(+)Ly-6G(+) cells and normal immune responses in Gal-9 transgenic mice. Our data uncover a novel mechanism by which the Tim-3/Gal-9 pathway regulates immune responses and identifies this pathway as a therapeutic target in diseases where myeloid-derived suppressor cells are disadvantageous.