We present a case of an osteosarcoma arising in ossifying fibromyxoid tumor. The patient was a 50-year-old man when an initial tumor was identified. It was a soft tissue mass in the left popliteal area, measuring 14x9 cm. The tumor was surgically removed. Histologically, the primary tumor had the appearance of a conventional ossifying fibromyxoid tumor, although there were cellular areas with pleomorphism and high mitotic rate. The neoplasm recurred 4 times over the next 8 years, involving underlying tissues, including skeletal muscle and bone. The recurrent lesions features areas of osteoid, which increased with each recurrence and in the fourth recurrence the tumor had an appearance of extraskeletal osteosarcoma lacking the ossifying fibromyxoid tumor. The tumor generalized that killed the patient with widespread metastatic disease.