An unknown-aged intact male Yorkshire terrier dog with a mass on the right caudoabdominal region was referred to the Veterinary Medical Teaching Hospital at Chungnam National University. Mass size was 4.5 cm in width, 6 cm in length and 3.5 cm in height. This case was suspected to seminoma by physical examination, laboratory examination, radiography, ultrasonography, CT and fine needle aspiration. This dog followed by treatment with surgical excision. After surgery, this case was diagnosed as seminoma by histopathological examination.