Osteosarcoma is a malignant, life-threatening tumor that affects children and adolescents. In this study, we identified high levels of calponin 3 (CNN3) protein in osteosarcoma tissues and cell lines. The receiver operating characteristic curve analysis revealed that CNN3 has diagnostic value for patients with osteosarcoma. We also found that high CNN3 expression was associated with tumor size, tumor stage, and lymph node and distant metastases. Moreover, high levels of CNN3 mRNA were associated with a poor overall survival rate and a shorter disease-free survival period. CNN3 silencing inhibited cell proliferation, induced apoptosis and cell cycle arrest at the G1 stage, and inhibited cell migration and invasion in vitro. Furthermore, CNN3 silencing also inhibited subcutaneous tumor growth and lung metastasis in vivo. Western blotting revealed that silencing of CNN3 resulted in downregulated expression of MMP9, VEGF, and vimentin, and upregulation of E-cadherin. CNN3 silencing also resulted in downregulation of the ERK1/2 and p38 signaling pathways. In conclusion, high CNN3 expression was found to help in the diagnosis of osteosarcoma, and was found to be associated with poor prognosis in patients. Therefore, CNN3 may play an oncogenic role during the progression of osteosarcoma by activating the ERK1/2 and p38 pathways.