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Mammary tumors in sheep are exceedingly rare, with all reported cases being benign to noninvasive and slow growing. Here, we describe a squamous cell carcinoma (SCC) with widespread metastases presumptively arising from the mammary parenchyma in a 3.5-y-old ewe. Following a history of chronic mastitis and severe swelling of the udder, the udder was removed surgically and submitted for examination. Three weeks after surgery, because of poor prognosis, the animal was euthanized and autopsied. The udder had no cutaneous lesions but had parenchymal disorganization and extensive areas of necrosis delimiting large, fluid-filled cavities. Two cord-like masses extended from the mammary gland into the pelvic cavity and subcutis of the right hindlimb. Numerous nodules were observed within the mammary and abdominal lymph nodes, peritoneum, lungs, and heart. Histologically, mammary and extramammary lesions consisted of neoplastic squamous epithelial cells, often surrounding keratinized cores. Neoplastic cells had diffuse cytoplasmic immunolabeling for pancytokeratin, confirming their epithelial lineage. Non-keratinized neoplastic cells were positive for p63 and, infrequently, for cytokeratin 19, suggesting mammary histogenesis. Considering the lack of a primary extramammary mass and the tumoral immunophenotype, a presumptive diagnosis was made of primary mammary SCC with metastasis to multiple organs. The aggressive clinical behavior of this neoplasm contrasts with that of reported ovine mammary neoplasms and cutaneous SCCs.