CD4(+) T cells are indispensable for optimal immunity to Mycobacterium tuberculosis (M.tb), a pathogen that triggers tuberculosis (TB) in humans. M.tb-specific human CD4(+) T cells are known to polarize toward an interferon-γ (IFN-γ)-producing, CCR4(-)CCR6(+)CXCR3(+)T-bet(+)RORγT(+) T helper 1* cell (T(H)1*cell) memory phenotype. We report that autosomal recessive deficiency of the human lymphocytic surface receptor LY9 (SLAMF3 and CD229), which is found in less than 10(-5) individuals in the general population, underlies TB in three unrelated patients due to selective impairment in IFN-γ production by T(H)1* cells. T(H)1* cells express higher levels of LY9 than other CD4(+) T cells. Mechanistically, LY9 polarizes naïve CD4(+) T cells toward memory T(H)1* cells by inducing T-bet via signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) and RORγT (thymus-specific retinoid-related orphan receptor γ) without SAP. LY9 costimulation enhances TCR-driven IFN-γ production of memory T(H)1*, but not T(H)1, cells in a T cell-intrinsic manner via NFAT1 (nuclear factor of activated T cells 1) and RORγT. LY9 is likely to govern an optimal T(H)1* cell- and IFN-γ-dependent protective immunity to M.tb in humans.