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Although tumor-infiltrating lymphocytes (TILs) maintain their ability to proliferate, persist, and eradicate tumors, they are frequently dysfunctional in situ. By performing both whole-genome CRISPR and metabolic inhibitor screens, we identify that nicotinamide phosphoribosyltransferase (NAMPT) is required for T cell activation. NAMPT is low in TILs, and its expression is controlled by the transcriptional factor Tubby (TUB), whose activity depends on the T cell receptor-phospholipase C gamma (TCR-PLCγ) signaling axis. The intracellular level of NAD+, whose synthesis is dependent on the NAMPT-mediated salvage pathway, is also decreased in TILs. Liquid chromatography-mass spectrometry (LC-MS) and isotopic labeling studies confirm that NAD+ depletion led to suppressed glycolysis, disrupted mitochondrial function, and dampened ATP synthesis. Excitingly, both adoptive CAR-T and anti-PD1 immune checkpoint blockade mouse models demonstrate that NAD+ supplementation enhanced the tumor-killing efficacy of T cells. Collectively, this study reveals that an impaired TCR-TUB-NAMPT-NAD+ axis leads to T cell dysfunction in the tumor microenvironment, and an over-the-counter nutrient supplement of NAD+ could boost T-cell-based immunotherapy. [Display omitted] • Levels of NAD+ and NAMPT are decreased in TILs compared to those of other T cells • Tubby is a transcriptional factor for NAMPT in T cells • NAMPT-mediated NAD+ production is essential for T cell activation by generating ATP • NAD+ supplementation enhances the tumor-killing function of T cells Wang et al. report that NAD+ biosynthesis through the TCR-PLCγ-TUB-NAMPT-mediated salvage pathway is important for T cell activation. Metabolically, NAD+ deficiency causes defects in aerobic glycolysis and oxidative phosphorylation. NAD+ supplementation rescues these defects and enhances the tumor-killing efficacy of T cells in both CAR-T and anti-PD1 mouse models. [ABSTRACT FROM AUTHOR]