A 36-year-old man was studied because of signs and symptoms of superior vena caval syndrome. Chest computerized tomography showed $10{ imes}8{ imes}6$ cm sized lobulated anterior mediastinal mass, compressing superior vena cava. Fine needle aspiration cytology revealed seminoma. There was no detectable tumor mass in the testes. We report a case of primary mediastinal seminoma presenting with superior vena caval syndrome.