Osteosarcoma is the most common primary bone tumor in humans, especially in childhood. However, thegenetic etiology for its pathogenesis remains elusive. It is known that microRNAs (miRNAs) are involved in thedevelopment of tumor progression. Here we show that microRNA-9 (miR-9) is a potential oncogene upregulatedin osteosarcoma cells. Knockdown of miR-9 in osteosarcoma resulted in suppressed colony formation and cellproliferation. Further study identified GCIP, a Grap2 and cyclin D interacting protein, as a direct target of miR-9. In addition, GCIP overexpression activated retinoblastoma 1 (Rb) and suppressed E2F transcriptional targetexpression in osteosarcoma cells. Moreover, GCIP depletion reversed miR-9 knockdown induced colony formationand cell proliferation suppression. In sum, these results highlight the importance of miR-9 as an oncogene inregulating the proliferation of osteosarcoma by directly targeting GCIP and may provide new insights into thepathogenesis of osteosarcoma.