Background & Aims: Transforming growth factor-β (TGF-β) has been implicated in the pathogenesis of liver disease. TGF-β is involved in liver regeneration and in the fibrotic and cirrhotic transformation with hepatitis viral infection. Hepatitis delta virus (HDV) infection causes fulminant hepatitis and liver cirrhosis. To elucidate the molecular mechanism of HDV pathogenesis, we examined the effects of HDV-encoded–only protein, the small hepatitis delta antigen (SHDAg), and the large hepatitis delta antigen (LHDAg), on TGF-β– and c-Jun–induced signaling cascades. Methods: The effects of either SHDAg or LHDAg on TGF-β– and c-Jun–induced signaling cascades in Huh7 and Cos7 cells were investigated by luciferase reporter gene assay, immunoprecipitation assay, electrophoretic mobility shift assay, Western blot analysis, and confocal microscopy analysis. Results: The LHDAg, but not the SHDAg, potentiated TGF-β– and c-Jun–induced signal activation, and the isoprenylation of LHDAg played a major role in signaling cascades. LHDAg synergistically activated hepatitis B virus X protein–mediated TGF-β and AP-1 signaling cascades. In addition, LHDAg enhanced the protein expression level of TGF-β–induced plasminogen activator inhibitor-1. Conclusions: LHDAg may induce liver fibrosis through the regulation of TGF-β–induced signal transductions. This regulation of TGF-β–mediated signaling is accomplished by the isoprenylation of LHDAg, which is a novel mechanism involved in HDV pathogenesis.