A 12-year-old male Yorkshire Terrier was presented because of decreased appetite. Physical examination revealed mammary gland swelling and galactorrhea. Contrast-enhanced computed tomographic scanning of the skull indicated an enlarged pituitary gland, compatible with a pituitary tumor. The serum prolactin concentration was markedly elevated. One week after the start of treatment with the dopamine agonist cabergoline, the serum prolactin concentration normalized and the galactorrhea resolved. Cabergoline was administered for approximately 4 months and then discontinued. Subsequently, serum prolactin concentration increased again, and mammary gland swelling and galactorrhea reappeared. The dog was euthanized 10 months after the first detection of the galactorrhea because of problems not directly related to pituitary disease. Postmortem examination revealed an infiltrative adenoma of the pituitary gland with immunolabeling for prolactin. The clinical and histopathologic findings indicated the diagnosis of a functional prolactinoma in a male dog.