Although the 5-year survival rate of osteosarcoma (OS) has risen to ~60-70%, a substantial portion of patients still respond poorly to chemotherapy and have a high risk of relapse or metastasis even after curative resection. In this study, we found that the expression of miR-142 was significantly reduced in OS tissues and OS cell lines, while Ras-related C3 botulinum toxin substrate 1 (Rac1) expression was increased in the OS tissues and OS cell lines compared with expression in the controls. We then demonstrated that miR-142 regulated Rac1 expression at the transcriptional and translational levels by directly targeting its 3'-untranslated region (3'UTR). In addition, by loss- and gain-of function experiments, we investigated the role of miR-142 in OS cell lines and found that miR-142 acted as a tumor suppressor in the OS cell lines and inhibited cell proliferation and cell invasion and arrested cell cycle in the S phase. Furthermore, miR-142 inhibited osteosarcoma cell invasion by inducing E-cadherin expression and reducing expression of matrix metalloproteinase 2 (MMP2) and MMP9. Thus, overexpression of miR-142 and/or knockdown of Rac1 would be a novel target for OS therapy in the future.