An adult male mixed breed dog developed pain, on swallowing, that lasted for 1 week. Physical exam and radiographs revealed a subcutaneous mass in the cervical area and three metastatic nodules in the lung. The cervical mass was surgically removed and fixed in 10% buffered formalin. Histopathologic diagnosis was carcinosarcoma supported by positive immunohistochemistry (IHC) results for cytokeratin and vimentin. IHC for thyroglobulin proved that the tissue of origin was the thyroid gland. This is the fifth canine case of thyroid carcinosarcoma to be documented, but the first one to be confirmed by specific cell markers. It is an extremely rare neoplasm that also occurs in people.