Interleukin (IL)-20 is a proinflammatory cytokine in the IL-10 family. IL-20 is associated with tumor promotion in the pathogenesis of oral, bladder, and breast cancer. However, little is known about the role of IL-20 in prostate cancer. We hypothesize that IL-20 promotes the growth of prostate cancer cells. Immunohistochemical staining showed that IL-20 and its receptors were expressed in human PC-3 and LNCaP prostate cancer cell lines and in prostate tumor tissue from 40 patients. In vitro, IL-20 upregulated N-cadherin, STAT3, vimentin, fibronectin, RANKL, cathepsin G, and cathepsin K, and increased the migration and colony formation of prostate cancer cells via activated p38, ERK1/2, AKT, and NF-κB signals in PC-3 cells. We investigated the effects of anti-IL-20 monoclonal antibody 7E on prostate tumor growth in vivo using SCID mouse subcutaneous and intratibial xenograft tumor models. In vivo, 7E reduced tumor growth, suppressed tumor-mediated osteolysis, and protected bone mineral density after intratibial injection of prostate cancer cells. We conclude that IL-20 is involved in the cell migration, colony formation, and tumor-induced osteolysis of prostate cancer. Therefore, IL-20 might be a novel target for treating prostate cancer.