We demonstrate that DNA hypomethylating agent (HMA) treatment can directly modulate the anti-tumor response and effector function of CD8(+) T cells. In vivo HMA treatment promotes CD8(+) T cell tumor infiltration and suppresses tumor growth via CD8(+) T cell-dependent activity. Ex vivo, HMAs enhance primary human CD8(+) T cell activation markers, effector cytokine production, and anti-tumor cytolytic activity. Epigenomic and transcriptomic profiling shows that HMAs vastly regulate T cell activation-related transcriptional networks, culminating with over-activation of NFATc1 short isoforms. Mechanistically, demethylation of an intragenic CpG island immediately downstream to the 3' UTR of the short isoform was associated with antisense transcription and alternative polyadenylation of NFATc1 short isoforms. High-dimensional single-cell mass cytometry analyses reveal a selective effect of HMAs on a subset of human CD8(+) T cell subpopulations, increasing both the number and abundance of a granzyme B-high, perforin(h)(ig)(h) effector subpopulation. Overall, our findings support the use of HMAs as a therapeutic strategy to boost anti-tumor immune response.