In recent years, the adenosine A_2A receptor (A_2AR) has shown exciting progress in the development of immunotherapies for the treatment of cancer. Herein, a 2-amino-7,9-dihydro-8H-purin-8-one compound (1) was identified as an A_2AR antagonist hit through in-house library screening. Extensive structure–activity relationship (SAR) studies led to the discovery of 2-aminopteridin-7(8H)-one derivatives, which showed high potencies on A_2AR in the cAMP assay. Compound 57 stood out with an IC₅₀ value of 8.3 ± 0.4 nM against A_2AR at the 5′-N-ethylcarboxamidoadenosine (NECA) level of 40 nM. The antagonistic effect of 57 was sustained even at a higher NECA concentration of 1 μM, which mimicked the adenosine level in the tumor microenvironment (TME). Importantly, 57 enhanced T cell activation in both the IL-2 production assay and the cancer-cell-killing model, thus demonstrating its potential as a lead for developing novel A_2AR antagonists in cancer immunotherapy.