As sufficient extracellular arginine is crucial for T cell function, depletion of extracellular arginine by elevated Arginase 1 (Arg1) activity has emerged as a hallmark immunosuppressive mechanism. However, the potential cell-autonomous roles of arginases in T cells have remained unexplored. Here we show that the arginase isoform expressed by T cells, the mitochondrial Arginase 2 (Arg2), is a cell-intrinsic regulator of CD8+ T cell activity. Both germ-line Arg2 deletion and adoptive transfer of Arg2-/- CD8+ T cells significantly reduced tumor growth in preclinical cancer models by enhancing CD8+ T cell activation, effector function and persistence. Transcriptomic, proteomic and high-dimensional flow cytometry characterization revealed a CD8+ T cell-intrinsic role of Arg2 in modulating T cell activation, anti-tumor cytoxicity and memory formation, independently of extracellular arginine availability. Furthermore, specific deletion of Arg2 in CD8+ T cells strongly synergized with PD-1 blockade for the control of tumor growth and animal survival. These observations coupled with the finding that pharmacologic arginase inhibition accelerates activation of ex vivo human T cells unveil Arg2 as a new therapeutic target for T cell-based cancer therapies.