The patient was a 25-year-old man. He was diagnosed as having a mediastinal tumor based on chest radiography. Chest computed tomography and magnetic resonance imaging revealed a cystic tumor in the anterior mediastinum. Total resection of the tumor with median sternotomy was performed. We anticipated that the tumor was a thymic cyst or teratoma, but pathological findings indicated a seminoma. The patient was discharged without any complication, and no recurrence of the lesion has been observed for 8 years after surgery without adjuvant therapy.