T cell senescence and exhaustion are major barriers to successful cancer immunotherapy. Here, we show that miR-155 increased CD8+ T cell antitumor function by restraining T cell senescence and functional exhaustion through epigenetic silencing of drivers of terminal differentiation. miR-155 enhanced Polycomb Repressor Complex 2 (PRC2) activity indirectly by promoting the expression of the PRC2 associated factor Phf19 through downregulation of the Akt inhibitor, Ship1. Phf19 orchestrated a transcriptional program extensively shared with miR-155 to restrain T cell senescence and sustain CD8+ T cell antitumor responses. These effects relied on Phf19 histone-binding capacity, which is critical for PRC2 recruitment to chromatin. These findings establish the miR-155–Phf19–PRC2 as a pivotal axis regulating CD8+ T cell differentiation, paving new ways for potentiating cancer immunotherapy through epigenetic reprogramming of CD8+ T cell fate. Overall design: Cells were sorted ex vivo and total RNA was isolated with a miRNeasy Mini kit (Qiagen). 10 ng total RNA was used for Nanostring analysis following the NanoString® nCounter®Expression CodeSet Design Manual. Background levels were calculated and subtracted from the samples, which were then normalized against the positive control and housekeeping gene probes.