An aged mongrel dog was admitted for hemimandibulectomy as treatment for a mandibular mass that had been diagnosed as osteosarcoma. The fibro-osseous mass that surrounded the first molar tooth and replaced alveolar and cortical bone was reclassified as ossifying fibroma on the basis of anatomic location and histologic features. The tumor was composed of isomorphic fusiform cells with few mitotic figures. Tumoral stroma contained trabeculae of woven bone that were bordered by a single layer of osteoblasts. Excision was deemed complete with no evidence of extension or metastasis by computed tomography of the skull or thoracic and abdominal radiography. The dog was reportedly healthy 6 months after initial presentation. Though far less common than osteosarcoma as a primary canine bone tumor, ossifying fibroma should be included in the differential diagnosis for fibro-osseous proliferations, especially those of the jaw. Although benign, en bloc excision may be necessary for surgical cure.