CD4 T cell help plays an important role in promoting CD8 T cell immunity to pathogens. In models of infection with vaccinia virus (VV) and Listeria monocytogenes, CD4 T cell help is critical for the survival of activated CD8 T cells during both the primary and memory recall responses. Still unclear, however, is how CD4 T cell help promotes CD8 T cell survival. In this study, we first showed that CD4 T cell help for the CD8 T cell response to VV infection was mediated by IL-21, a cytokine produced predominantly by activated CD4 T cells, and that direct action of IL-21 on CD8 T cells was critical for the VV-specific CD8 T cell response in vivo. We next demonstrated that this intrinsic IL-21 signaling was essential for the survival of activated CD8 T cells and the generation of long-lived memory cells. We further revealed that IL-21 promoted CD8 T cell survival in a mechanism dependent on activation of the STAT1 and STAT3 pathways and subsequent upregulation of the prosurvival molecules Bcl-2 and Bcl-x L. These results identify a critical role for intrinsic IL-21 signaling in CD8 T cell responses to an acute viral infection in vivo and may help design effective vaccine strategies.