A 27-year-old Thoroughbred gelding was examined for a right nasal mass visible inside the right nares. Airflow through the right nostril was absent. Endoscopy and radiography revealed the mass to occupy the entire right nasal passage. Nasal biopsies were inconclusive, so en bloc resection was performed. A diagnosis of an incompletely resected osteoblastic osteosarcoma was made. Endoscopic biopsies performed 4 weeks post surgery revealed osteosarcoma cells present in the caudal right nasal cavity. Metastatic disease was not present in mandibular lymph node aspirates or on thoracic radiographs. The right nasal passage was irradiated with 12 treatments over the course of 4 weeks. Comfort and quality of life were excellent during treatment and no adverse side effects were noted. Endoscopy and follow-up biopsies at 1, 2, 4, 12 and 14 months post radiation therapy have not found any evidence of regrowth of the osteosarcoma.