Knockdown of MED19 by lentivirus-mediated shRNA in human osteosarcoma cells inhibits cell proliferation by inducing cell cycle arrest in the G0/G1 phase. MED 19 is a subunit of the mediator complex, which is a coactivator of RNA polymerase II and also interacts with the downstream coding region of many genes. However, the role of MED19 in osteosarcoma is unknown. In the present study, we applied lentivirus-mediated short hairpin RNA (shRNA)-triggered RNA interference to downregulate MED19 expression in human osteosarcoma SaOS-2 and U2OS cells. Knockdown of MED19 expression was confirmed by real-time PCR and Western blot. It was found that silencing of MED19 resulted in decreased cell viability, colony formation capacity, and DNA synthesis ability in both cells, as well as the G0/G1 phase cell cycle arrest. These results implied that MED19 played an important role in cell growth and cell cycle progression of human osteosarcoma cells. MED19 may be an attractive candidate for the therapeutic target in osteosarcoma.