<p>This research explored the role of microRNA (miRNA)-21 in prostate cancer (PCa) cells, as well as its regulation of the JAK/STAT pathway in PCa cells. Quantitative real-time PCR was employed to examine miRNA-21 expression in PCa cells. Cell viability and proliferation were detected by MTT and colony formation assays. Cell migration was measured by wound healing and transwell assays. The janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway-related protein expression was detected using western blot. The results indicated that miRNA-21 was significantly up-regulated in PCa cells, and inhibition of miRNA-21 suppressed the viability, proliferation and migration of PCa cells. Besides, inhibition of miRNA-21 lessened the levels of JAK/STAT pathway-related proteins in both PCa cells. Additionally, Ruxolitinib treatment (an inhibitor of the JAK/STAT pathway) could reverse the elevated cell viability and proliferation in miRNA-21 mimics-transfected PCa cells. Taken together, our study demonstrates that miRNA-21 promotes the migration and proliferation of PCa cells via activating the JAK /STAT pathway.</p>