MicroRNAs (miRNAs) play important roles in osteoclastogenesis and bone resorption. However, no study has investigated the role of miRNA in postmenopausal osteoporosis. Here, we report that miR-503 was markedly reduced in circulating progenitors of osteoclasts–CD14+ peripheral blood mononuclear cells (PBMCs) from postmenopausal osteoporosis patients compared with those from postmenopausal healthy women. Overexpression of miR-503 in CD14+ PBMCs inhibited receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis. Conversely, silencing of miR-503 in CD14+ PBMCs promoted osteoclastogenesis. RANK, which is activated by the binding of RANKL and inducing osteoclast differentiation, was confirmed to be a target of miR-503. In vivo, silencing of miR-503 using a specific antagomir in ovariectomy (OVX) mice increased RANK protein expression, promoted bone resorption, and decreased bone mass, whereas overexpression of miR-503 with agomir inhibited bone resorption and prevented bone loss in OVX mice. Thus, our study revealed that miR-503 plays an important role in the pathogenesis of postmenopausal osteoporosis and contributes to a new therapeutic way for osteoporosis. © 2014 American Society for Bone and Mineral Research.