A 53-year-old woman had extraskeletal osteosarcoma that developed from a soft tissue bony mass present on the volar aspect of the left wrist for 4 years. Initially, the bony mass was soft and movable, but during the first year it became hard and fixed. The patient had no history of trauma. Because the lesion did not grow or cause any symptoms, the patient did not come to the hospital until 4 years after she first noticed the lesion. Radiologically, the bony mass had features characteristic of mature myositis ossificans, showing "eggshell" ossification. A nonmineralized soft tissue mass occurred between the surface of the radius and the bony shell. Histologically, a high-grade osteosarcoma was present between the surface of the radius and the well-differentiated bone tissue, which included fatty and hematopoietic marrow. All the findings indicated that our patient had an extremely rare case of malignant transformation of myositis ossificans.