A 13-year-old castrated male Labrador retriever dog presented with a mass caudal to the first molar of his left mandible. Although the tumor was excised, a recurrent tumor was detected one month later and resected. Both tumors displayed invasive growth and were composed of neoplastic proliferation arranged in irregular lobules, nests and cords continuous with mucosal epithelium. The most prominent feature of the tumors was the presence of many proliferating spindle cells admixed with palisading basal-like cells, acanthocytes and stellate cells. In immunohistochemical examinations, the spindle cells were found to be positive for vimentin; cytokeratin AE1/AE3, 5/6, 14 and 19; and p63. The other neoplastic cells were positive for all of these markers shown above except vimentin. Based on these findings, the tumors were diagnosed as spindle cell ameloblastic carcinoma.