A 9-year-old male poodle dog exhibited a white to yellow 3.0$ imes$2.0$ imes$3.9 cm mass in the left testicle with cryptorchidism and monorchidism. Grossly, necrosis and hemorrhage were observed on the cut surface of the mass. Histopathologically the mass consisted of seminoma and sertoli cell tumor (SCT). Seminoma was characterized with diffuse solitary pattern in which tumor cells had polygonal nuclei and clear cell border. SCT was composed of compact neoplastic cells with small nuclei and indefinite cytoplasm. The tumor was occupied mainly with SCT. SCT and seminoma were observed occasionally in the same lesion. From these results, this case was diagnosed as concurrent seminoma and SCT in the left cryptorchidism and monorchidism. To our knowledge, this is the first report of concurrent occurrence of seminoma and SCT in a dog with cryptorchidism and monorchidism in Korea.