Biphasic neoplasms with a benign to atypical epithelial component and a usually low-grade malignant stromal component have been reported in various sites, probably being best known for their occurrence in the uterine corpus (mullerian adenosarcoma). We report a tumor of this type that occurred in the testis of a 76-year-old man and, to our knowledge, is the first mesodermal adenosarcoma reported at this site. The patient had scrotal swelling for many years with a pronounced increase in the swelling over the past 2 years. A large complex solid-cystic testicular tumor was evident on ultrasound, and examination of a radical orchiectomy specimen showed a 6.5-cm mass. On microscopic examination, the neoplasm had a phyllodes-like appearance with bland cuboidal to flattened epithelium covering polypoid fronds, and lining glands and cysts. The stroma varied from cellular, particularly where it condensed around the glands and cysts, to hypocellular and hyalinized. It was immunoreactive for muscle specific actin, CD10, and to a lesser degree, desmin. This case expands the known sites where mesodermal adenosarcoma may occur. The histogenesis is speculative, but possible options are discussed.