Tumor hypoxia–driven accumulation of extracellular adenosine was shown to facilitate tumor evasion by engaging the immunosuppressive, intracellular cAMP-elevating A 2 adenosine receptors (A 2 R) on tumor-reactive effector T cells, but there remains a need for careful evaluation of the limiting factors and properties of A 2 R blockade–enabled antitumor immunity. In studies of A 2A R and/or A 2B R gene–deficient mice, we found that A 2A R deletion—but not A 2B R deletion—liberates endogenous CD8+ T cell antitumor immunity against weakly immunogenic MCA205 sarcomas. Studies of adoptively transferred A 2A R−/−, A 2B R−/−, or A 2A R−/−/A 2B R−/− tumor-reactive T cells confirmed that immunosuppression in the tumor microenvironment was mediated by A 2A R on CD8+ T cells. Treatment with A 2A R antagonist mimicked A 2A R gene deletion in adoptive T cell immunotherapy. This therapeutic benefit of targeting A 2A R was independent of the anatomical location of tumor growth. The enhanced antitumor reactivity also led to the eradication of established intracranial tumors, which was associated with mouse survival and the maintenance of long-lasting, tumor-specific immunological memory. The blockade of the A 2A R on adoptively transferred T cells by synthetic A 2A R antagonist led to higher levels of IFN-γ secretion by tumor-infiltrating CD8+ T cells. These data clarify the mechanism of hypoxia-driven immunosuppression in the tumor microenvironment by A 2A R on tumor-reactive CD8+ T cells and show that selective A 2A R antagonists can be effective in improving the outcomes of T cell–based immunotherapies. Demonstration of the T cell dose dependency of tumor rejection points to a major limitation of current cancer immunotherapies, in which the presence of sufficient numbers of tumor-reactive T cells in a patient is not known.