Natural killer (NK) cells are innate lymphocytes that display features of adaptive immunity during viral infection. Biallelic mutations in IRF8 have been reported to cause familial NK cell deficiency and susceptibility to severe viral infection in humans; however, the precise role of this transcription factor in regulating NK cell function remains unknown. Here, we show that cell-intrinsic IRF8 was required for NK-cell-mediated protection against mouse cytomegalovirus infection. During viral exposure, NK cells upregulated IRF8 through interleukin-12 (IL-12) signaling and the transcription factor STAT4, which promoted epigenetic remodeling of the Irf8 locus. Moreover, IRF8 facilitated the proliferative burst of virus-specific NK cells by promoting expression of cell-cycle genes and directly controlling Zbtb32, a master regulator of virus-driven NK cell proliferation. These findings identify the function and cell-type-specific regulation of IRF8 in NK-cell-mediated antiviral immunity and provide a mechanistic understanding of viral susceptibility in patients with IRF8 mutations. Display omitted •IRF8 is required for NK-cell-mediated protection against MCMV•Rapid NK cell upregulation of IRF8 during MCMV infection is STAT4 dependent•IRF8 is essential for MCMV-specific NK cell expansion•IRF8 supports NK cell proliferation by directly regulating Zbtb32 The link between human IRF8 mutations and immunodeficiency is poorly understood. Adams et al. demonstrate that IRF8 is required for NK-cell-mediated antiviral immunity by promoting proliferation of virus-specific NK cells.