question: Members of the Pegivirus genus, family Flaviridiae, widely infect humans and other mammals, including nonhuman primates, bats, horses, pigs, and rodents, but are not associated with disease. Here, we report a new, genetically distinct pegivirus in goose (Anser cygnoides), the first identified in a non-mammalian host species. Goose pegivirus (GPgV) can be propagated in goslings, embryonated goose eggs, and primary goose embryo fibroblasts and is thus the first pegivirus that can be efficiently cultured in vitro Experimental infection of GPgV in goslings via intravenous injection revealed robust replication and shown high lymphotropism. Analysis of the tissue tropism of GPgV revealed that the spleen and thymus were the organs bearing the highest viral loads. Importantly, GPgV could promote clinical manifestations of goose parvovirus infection, reducing weight gain and 7% mortality. This finding contrasts with the lack of pathogenicity that is characteristic of previously reported pegiviruses.IMPORTANCE Members of the Pegivirus genus, family Flaviviridae, widely infect humans and other mammals, but are described as causing persistent infection and lacking pathogenicity. The efficiency of in vitro replication system for pegivirus is poor limiting investigation into viral replication steps. Because of that, the pathogenesis, cellular tropism, route of transmission, biology, and epidemiology of pegiviruses remain largely uncovered. Here, we report a phylogenetically distinct goose pegivirus (GPgV) which should be classified as a new species. GPgV proliferated in cell culture in a species- and cell-type specific manner. Animal experiments shown GPgV lympho-tropism and promotes goose parvovirus clinical manifestations. This study provides the first cell culture model for pegivirus, opening new possibilities for studies of pegivirus molecular biology. More importantly, our finding stand in contrast to the lack of identified pathogenicity of previously reported pegiviruses, which sheds lights on the pathobiology of pegivirus.