Primary mediastinal seminoma is an uncommon tumor usually located in the anterior mediastinum. The majority of cases occur in young males. We report here an extremely rare case of a 69-year-old male with primary seminoma in the middle mediastinum. The patient had no complaints, but an abnormal shadow was seen in a routine chest X-ray. We performed a tumorectomy in the middle mediastinum, a thymectomy and an orchidectomy and added postoperative chemotherapy. It seems that the tumor was not associated with the thymus, so we believe the tumor did not stem from the embryonic thymus. Our case demonstrated that mediastinal seminoma does not always occur in the anterior mediastinum of young males. Although this case is rare, seminoma should be included among the possible diagnoses of a middle mediastinal mass.