Metformin, a prescribed drug for type 2 diabetes, has been reported to have anti-cancer effects; however, the underlying mechanism is poorly understood. Here we show that this mechanism may be immune-mediated. Metformin enabled normal but not T-cell–deficient SCID mice to reject solid tumors. In addition, it increased the number of CD8⁺ tumor-infiltrating lymphocytes (TILs) and protected them from apoptosis and exhaustion characterized by decreased production of IL-2, TNFα, and IFNγ. CD8⁺ TILs capable of producing multiple cytokines were mainly PD-1⁻Tim-3⁺, an effector memory subset responsible for tumor rejection. Combined use of metformin and cancer vaccine improved CD8⁺ TIL multifunctionality. The adoptive transfer of antigen-specific CD8⁺ T cells treated with metformin concentrations as low as 10 μM showed efficient migration into tumors while maintaining multifunctionality in a manner sensitive to the AMP-activated protein kinase (AMPK) inhibitor compound C. Therefore, a direct effect of metformin on CD8⁺ T cells is critical for protection against the inevitable functional exhaustion in the tumor microenvironment.