Osteosarcoma is the most common primary malignant bone tumor in children and adolescents. Overactive EGFR signaling is frequently seen in osteosarcoma cells, and represents a potential therapeutic target. However, feedback activation of STAT3 after EGFR inhibition is linked to treatment resistance, suggesting that combined EGFR/STAT3 inhibition may be needed to overcome this effect. Cantharidin and its analogues have shown strong anticancer effects, including STAT3 inhibition, in several tumor cells. Therefore, we investigated the effects of sodium cantharidate (SC), either as monotherapy and in combination with the EGFR inhibitor erlotinib, on STAT3 activation and osteosarcoma cell growth. Cell viability, migration, and apoptosis assays were performed in human MG63 and U2OS cells, and MG63 xenografts were generated in nude mice to verify the suppression of tumor growth in vivo. Additionally, western blotting and immunohistochemistry were used to verify the STAT3 and EGFR phosphorylation statuses in xenografts. We found that SC repressed cell viability and migration and induced apoptosis in vitro, while combined SC and erlotinib treatment enhanced osteosarcoma growth suppression by preventing feedback activation of STAT3. These data support further development of cantharidin-based combination therapies for metastatic and recurrent/refractory osteosarcoma.