Osteosarcoma is a malignant bone cancer that mainly affects children. SOX9 plays a key role in bone formation and osteosarcoma, and Claudin-8 (CLDN8), a tight junction protein, contributes to proliferation of osteosarcoma cells. The aim of this study was to investigate the relationship between SOX9 and CLDN8 in osteosarcoma. The expression levels of SOX9 and CLDN8 were determined in osteosarcoma specimens (n = 25) by qRT-PCR and Western blot analyses. The levels of SOX9 mRNA and protein were significantly higher in osteosarcoma tissues than those in adjacent non-tumor tissues, whereas the expression levels of CLDN8 mRNA and protein were significantly lower in osteosarcoma tissues. Immunohistochemical analysis showed the high expression of SOX9 in 56 out of 97 osteosarcoma tissues (57.7%). By contrast, the low expression of immunoreactive CLDN8 was observed in 62 of 97 osteosarcoma tissues (63.9%). There was the inverse correlation between the expression levels of SOX9 and CLDN8 proteins (R = -0.633, P < 0.001). The overall survival was poorer in the patients with high SOX9 expression. Subsequently, using two human osteosarcoma cell lines, we showed that knockdown of SOX9 inhibited cell proliferation and migration but promoted cell apoptosis. Importantly, knockdown of SOX9 increased the expression levels of CLDN8 protein. The transient luciferase-reporter assays suggest that SOX9 may inhibit the promoter activity of the CLDN8 gene in osteosarcoma cells. In conclusion, we provide the evidence demonstrating that SOX9 may promote cell growth by repressing the expression of CLDN8. Thus, SOX9 may be a therapeutic target for osteosarcoma.