Intrahepatic cell-derived, early IL-17 is important for activating antigen-presenting cells in viral infection; however, the source and regulation of this IL-17 surge in the liver microenvironment are not well defined. Here, we present evidence for a significant expansion of IL-17A/F-producing cells in mouse liver within 24 h of adenovirus (Ad) infection. In addition to λδ T cells, a subset of IL-17A/F+ cells expressed no myeloid or lymphoid lineage markers. Instead, they expressed high levels of stem cell markers, IL-7 receptor and RORλt, consistent with the newly described innate lymphoid cells. Based on their unique surface markers and cytokine profiles, these cells were confirmed as group 3 innate lymphoid cells (ILC3s). In addition to Ad infection, ILC3s were also found in mouse liver within 24 h of LCMV infection. They contributed significantly to the establishment of the early cytokine milieu in virus-infected liver. Functional studies with mice deficient of IL-17 receptor, IL-17A, and IL-17F further revealed that IL-17 signaling was critical for priming T cell responses in viral hepatitis. IL-17A repressed IL-17F secretion in vitro and in vivo; IL-17F+ intrahepatic cells expanded more vigorously in IL-17A knockout animals, permitting efficient antigen-presentation and T cell function. However, IL-17F neither inhibited IL-17A in vitro, nor regulated its secretion in vivo. Together, this study has demonstrated the importance of a unique intrahepatic subpopulation and subsequent IL-17A/F regulation at initial stages of viral infection in the liver. These results have important implications for anti-cytokine biologic therapy and vaccine development.