A 3-year-old, neutered, male Golden Retriever was presented for evaluation of a 10 X 9 X 5 mm, firm, red, raised, cutaneous mass located over the left cranial thorax and noted incidentally by the owner. On cytologic evaluation of a fine-needle aspirate of the mass, the interpretation was a malignant tumor with predominantly mesenchymal features. Differentials included liposarcoma, atypical amelanotic melanoma, anaplastic sarcoma, and anaplastic carcinoma. Following complete excision of the mass, a diagnosis of sebaceous adenocarcinoma was made based on histologic features, positive immunostaining for pancytokeratin, and negative staining for vimentin, Melan-A, and S-100. There was no evidence of metastasis on physical examination or thoracic radiographs, and the prognosis was good. The unique and previously unreported cytologic features of this small, sebaceous adenocarcinoma were the extreme pleomorphism, including marked anisocytosis, anisokaryosis, and multinuclearity, and the paucity of epithelial features.