SUMMARY Upon infection, CD8+ T cells undergo a stepwise process of early activation, expansion and differentiation into effector cells. How these phases are transcriptionally regulated is incompletely defined. Here, we report that interferon regulatory factor 4 (IRF4), dispensable for early CD8+ T cell activation, was vital for sustaining the expansion and effector differentiation of CD8+ T cells. Mechanistically, IRF4 promoted the expression and function of Blimp1 and T-bet, two transcription factors required for CD8+ T cell effector differentiation, while repressed genes that mediate cell cycle arrest and apoptosis. Selective ablation of Irf4 in peripheral CD8+ T cells impaired anti-viral CD8+ T cell responses, viral clearance and CD8+ T cell-mediated host recovery from influenza infection. IRF4 expression was regulated by T cell receptor (TCR) signaling strength via mammalian target of rapamycin (mTOR). Our data reveal that IRF4 translates differential strength of TCR-signaling into different quantitative and qualitative CD8+ T cell responses.