We found upregulation of expression of the microRNA miR-155 in primary effector and effector memory CD8+ T cells, but low miR-155 expression in naive and central memory cells. Antiviral CD8+ T cell responses and viral clearance were impaired in miR-155-deficient mice, and this defect was intrinsic to CD8+ T cells, as miR-155-deficient CD8+ T cells mounted greatly diminished primary and memory responses. Conversely, miR-155 overexpression augmented antiviral CD8+ T cell responses in vivo. Gene-expression profiling showed that miR-155-deficient CD8+ T cells had enhanced type I interferon signaling and were more susceptible to interferon's antiproliferative effect. Inhibition of the type I interferon–associated transcription factors STAT1 or IRF7 resulted in enhanced responses of miR-155-deficient CD8+ T cells in vivo. We have thus identified a previously unknown role for miR-155 in regulating responsiveness to interferon and CD8+ T cell responses to pathogens in vivo.