We present a rare case of malignant invasive thymoma (type A) arising from the posterior mediastinum. A 17-year-old girl was referred to our clinic after detection of a mass on a chest roentgenogram. Thoracoscopy showed a lobulated, pale yellowish mass located in the posterior compartment together with several foci in the lung parenchyma. The tumor and parenchymal foci were totally resected through a left minithoracotomy. Postoperative pathology revealed malignant invasive thymoma type A.