Osteosarcoma (OS) has become one of the most common primary malignant tumors in the children and adolescents with a poor prognosis mainly due to high metastasis. A disintegrin and metalloprotease 9 (ADAM-9) plays a role in tumorigenesis, invasion, and metastasis in several tumors. miR-126 has been reported to be downregulated in OS tumor. However, the involvement of ADAM-9 in the pathology of OS and the relationship between miR-126 and ADAM-9 in OS cells remain unclear. In this study, using quantitative reverse-transcribed PCR (qRT-PCR) analysis on 37 pairs of OS tumors and matched adjacent normal bone tissues, we found that ADAM-9 is significantly upregulated, while miR-126 is downregulated in human OS tumors. Association analysis revealed that upregulation of ADAM-9 and downregulation of miR-126 are significantly involved in advanced clinical stage development and distant metastasis. Luciferase reporter assay revealed that miR-126 could directly target ADAM-9 3' untranslated region (UTR) and inhibit its expression in U2OS and MG-63 cells. Functional experiments revealed that downregulating ADAM-9 by miR-126 inhibited cellular growth, invasion, and migration in U2OS and MG-63 cells. In rescue experiments, restored ADAM-9 expression attenuated miR-126-mediated suppression, while knockdown of ADAM-9 by small interfering RNA (siRNA) represented similar results with miR-126-mediated tumor suppression in U2OS cells. Taken together, our data indicated that miR-126 inhibits cell growth, invasion, and migration of OS cells by downregulating ADAM-9.