JARID1B has been proven to be upregulated in many human malignancies and is correlated with tumor progression. However, its expression and clinical significance in osteosarcoma are still unclear. Thus, the aim of this study was to explore the effects of JARID1B in osteosarcoma tumorigenesis and development. In this study, we found that the expression levels of JARID1B in osteosarcoma tissues were significantly higher than those in corresponding noncancerous bone tissues. In addition, JARID1B upregulation occurred more frequently in osteosarcoma specimens from patients with a poor prognosis. After JARID1B transfection in osteosarcoma cells, cell proliferation was significantly promoted in vitro and in vivo. On the contrary, knockdown of JARID1B inhibited cell proliferation in vitro and tumor growth in vivo. JARID1B can also decrease the G0/G1 phase cell numbers and increase the S and G₂/M phase cell numbers. We further demonstrated that JARID1B regulates cyclin D1 expression through H3K27me3. These findings indicate that JARID1B may act not only as a novel diagnostic and prognostic marker but also as a potential target for molecular therapy in osteosarcoma.