Several cellular signaling pathways, including insulin/IGF signaling, are known to be activated in hepatocellular carcinoma (HCC). Here, we investigated the roles of insulin receptor substrate (Irs) 1 and Irs2, both of which are the major molecules to be responsible for transducing insulin/IGF signaling in the liver, in the development of HCC by inducing chemical carcinogenesis using diethylnitrosamine (DEN) in mice. The Irs1 mRNA and protein expressions were upregulated in the tumors, along with enhanced insulin signaling. Liver-specific Irs1-knockout (LIrs1KO) mice exhibited suppression of DEN-induced HCC development, accompanied by reduced cancer cell proliferative activity and reduced activation of Akt. Gene expression analyses revealed that the tumors in the DEN-treated LIrs1KO mice showed modest metabolic alterations during hepatocarcinogenesis as well as decreased inflammation and invasion potentials. On the other hand, liver-specific Irs2-knockout (LIrs2KO) mice showed a similar pattern of HCC development to the DEN-treated control wild-type mice. Based on the knowledge that Wnt/β-catenin signaling is activated in HCC, we focused on Wnt/β-catenin signaling and demonstrated that Irs1 expression was induced by Wnt3a stimulation in the primary hepatocytes, associated with insulin-stimulated Akt activation. These data suggest that upregulated Irs1 by Wnt/β-catenin signaling plays a crucial role in the progression of HCC.