The success of poxviruses as pathogens depends upon their extensive antagonism of host immune responses by an arsenal of immunomodulatory proteins. The study of these virulence factors in their natural host reveals not only the mechanisms of subversion by which pathogens cause disease but also the complex structure of immune defenses that function to protect the host. The C15 protein of ectromelia virus (ECTV, the agent of mousepox) is the largest of the ECTV immunomodulatory proteins and is a member of a well-conserved poxviral family that contributes profoundly to virulence and has previously been studied as inhibitors of T cell activation. Here, I reveal that C15 also impacts early viral replication and spread in vivo at a time prior to T cell-mediated control of ECTV. I show that this early pro-viral effect of C15 is independent of , CD4 and CD8 T cells and dependent upon natural killer (NK) cells. I demonstrate that in vivo the NK cell response to ECTV infection is restricted by C15 and that the antagonism is selective to NK cell cytolytic function and not cytokine production. This function is recapitulated in vitro, but was not due to the inhibition of C15 on the transcription of any factors related to NK cell recruitment or activation. I conclude that in addition to its previously identified capacity to antagonize traditional T cell receptor-dependent CD4 T cell activation, C15 inhibits NK cell cytolytic function resulting in increased viral replication and dissemination. Unexpectedly, I also demonstrate that, while this does not contribute to control of replication at this early time, C15 appears to be capable of similarly impacting the antigen-independent bystander function of CD4 and CD8 T cells, namely inhibiting T cell cytolytic function but not cytokine production. Together, this work reveals the intricate interactions of a single viral protein with various lymphocytes throughout the course of infection and builds on a body of literature demonstrating the importance of NK cells in early restriction of virus within the draining lymph node.