A 12-year-old Peruvian Paso stallion was examined for scrotal swelling and unilateral testicular enlargement of 4–6 weeks duration. Physical examination and ultrasonography of the scrotum revealed an enlarged, nonpainful right testis with a well encapsulated mass at the cranial-medial aspect containing echodense flocculent material. Unilateral castration was performed and the testis, spermatic cord and vaginal tunic were submitted for histological examination, which revealed a mastocytoma with accompanying evidence of malignancy. Short-term follow-up examination did not reveal any evidence of abdominal metastasis and the stallion has since successfully achieved pregnancy in a small number of mares. Although considered rare, mast cell neoplasia should be considered as a differential diagnosis when unilateral testicular enlargement is encountered.