Thoracic venous aneurysms are extremely rare conditions. Only 14 innominate venous aneurysms have been reported in the literature. We report a case of a 13-year-old girl who presented with a mediastinal mass on chest roentgenogram, which was performed because the patient was postoperatively tachypneic after a routine appendectomy. Further investigations revealed a large left innominate venous aneurysm that was growing rapidly. The patient underwent aneurysmectomy and reconstruction of the innominate vein. The patient's postoperative course was uneventful, and her respiratory symptoms resolved.