Osteosarcoma is the most common primary sarcoma of bone. The use of Chitooligosaccharide (COS) as a drug carrier is an emerging new strategy for cancer therapy. However, the application of COS in osteosarcoma has not been reported. Here, we investigated the influence of COS on osteosarcoma, and suggested the underlying mechanism. Initially, we obtained COS with a low-degree-polymerized (DP = 2-6) by enzymatic hydrolysis. Using these COS materials, in vitro assays showed that COS elicited the anti-tumor activity against osteosarcoma cells. We found that COS had significant effects on cell growth, metastasis inhibition, apoptosis and autophagy induction, and triggered pro-apoptosis autophagy through p53/mTOR signaling pathway in osteosarcoma cells. In addition, the COS also inhibited tumor growth and metastasis in an osteosarcoma xenograft model in vivo. Finally, we showed that COS could increase sensitivity to chemotherapy of cisplatin in vitro. Thus, we provide experimental evidence to demonstrate that COS has anti-tumor effect on osteosarcoma, and COS can be a new potential therapeutic candidate for the treatment of osteosarcoma.