Hydrogen peroxide (H2O2) induces oxidative injury to human osteoblasts. The expression and potential function of circular RNA HIPK3 (circHIPK3) in H2O2-treated human osteoblasts were tested. We show that H2O2 significantly downregulated circHIPK3 in OB-6 cells and primary human osteoblasts. Furthermore, circHIPK3 levels were decreased in the necrotic femoral head tissues of dexamethasone-treated patients. In OB-6 osteoblastic cells and primary human osteoblasts, forced overexpression of circHIPK3 by a lentiviral construct alleviated H2O2-induced viability reduction, cell death and apoptosis. Contrarily, circHIPK3 silencing by targeted shRNA potentiated H2O2-induced cytotoxicity in OB-6 cells and primary human osteoblasts. Moreover, circHIPK3 downregulation by H2O2 induced miR-124 accumulation in OB-6 cells and primary human osteoblasts. On the contrary, miR-124 inhibition by transfection of the miR-124 inhibitor protected human osteoblasts from H2O2. Importantly, forced overexpression of miR-124 by transfection of the miR-124 mimic induced significant cytotoxicity in OB-6 cells and primary human osteoblasts. H2O2 downregulated miR-124's targets, cyclin dependent kinase 6 and Rho-Associated Protein Kinase 1, in human osteoblasts. In conclusion circHIPK3 downregulation mediates H2O2-induced cytotoxicity in human osteoblasts.