A mucinous adenocarcinoma of the thymus is rare, and there is presently limited information on the clinical aspects of this uncommon type of tumor. Only six cases have been reported in the literature. A 49-year-old Japanese male presented with an anterior mediastinal mass, and underwent a tumor resection. Histologically the tumor consisted of a mucinous adenocarcinoma with pleural dissemination. Postoperative radiotherapy and chemotherapy were performed. The patient is still alive with pleural dissemination 11 months after surgery.