Understanding the mechanisms triggering hepatogenic differentiation of stem/progenitor cells would be useful for studying postnatal liver regeneration and development of liver cell therapies. Many evidences support the involvement of Sox9 transcription factor in liver development. Here, we investigate the possibility of liver mesenchymal stem/progenitor cells to constitutively express Sox9 by using reverse transcription-quantitative polymerase chain reaction, immunocytochemistry, and western blotting. The involvement of Sox9 in hepatogenic differentiation was assessed by following its expression at different steps of the process, evaluating the impact of its altered expression, and analyzing its expression in human liver disease specimen. Liver mesenchymal stem/progenitor cells constitutively express Sox9 at both the mRNA and protein levels. Upon hepatogenic differentiation, Sox9 expression is downregulated mainly in the maturation step after oncostatin M treatment. Induction of Sox9 expression using transforming growth factor beta is accompanied with a decrease of the quality of hepatogenic differentiation. Blunting Sox9 expression using specific ShRNA clearly alters the levels of several hepatic markers, an effect confirmed in HepG2 cells. In human liver disease specimen, Sox9 expression is enhanced at both the mRNA and protein levels compared with healthy donors. The current data demonstrate that Sox9 may play a pivotal role in hepatocyte lineage development, including adult liver mesenchymal stem/progenitor cells. Further studies on the identification of pathways regulated by or regulating Sox9 will certainly gain insight into the molecular networks controlling hepatogenic differentiation.