Osteosarcoma is a common primary malignant bone tumor, the cure rate of which has stagnated over the past 25–30 years. Arsenic sulfide (As2S2), the main active ingredient of the traditional Chinese medicine realgar, has been proved to have antitumor efficacy in several tumor types including acute promyelocytic leukemia, gastric cancer and colon cancer. Here, we investigated the efficacy and mechanism of As2S2 in osteosarcoma both in vitro and in vivo. In this study, we demonstrated that As2S2 potently suppressed cell proliferation by inducing G2/M phase arrest in various osteosarcoma cell lines. Also, treatment with As2S2 induced apoptosis and autophagy in osteosarcoma cells. The apoptosis induction was related to PARP cleavage and activation of caspase-3, −8, −9. As2S2 was demonstrated to induce autophagy as evidenced by formation of autophagosome and accumulation of LC3II. Further studies showed that As2S2-induced apoptosis and autophagy could be significantly attenuated by ROS scavenger and JNK inhibitor. Moreover, we found that As2S2 inhibited Akt/mTOR signaling pathway, and suppressing Akt and mTOR kinases activity can increase As2S2-induced apoptosis and autophagy. Finally, As2S2in vivo suppressed tumor growth with few side effects. In summary, our results revealed that As2S2 induced G2/M phase arrest, apoptosis, and autophagy via activing ROS/JNK and blocking Akt/mTOR signaling pathway in human osteosarcoma cells. Arsenic sulfide may be a potential clinical antitumor drugs targeting osteosarcoma.