A 14-year-old Arabian gelding was examined for a mass on the lateral aspect of the left tarsus and proximal lateral metatarsus. Radiography revealed a predominately osteolytic lesion involving the fourth tarsal bone and the proximal third and fourth metatarsal bone. Results of histopathological examination of the mass prior to surgery were consistent with an undifferentiated sarcoma. The mass was surgically excised and the remaining bone defect treated with absorbable beads containing cisplatin. Histology and immunohistochemistry of the mass removed at surgery revealed a telangiectatic osteosarcoma. The surgical site healed uneventfully and the horse was performing as a low level jumper 8.5 years after surgery.