Thymic carcinoma is a rare tumor. The most common histological subtype is squamous cell carcinoma, and only a few cases of thymic adenocarcinoma have been reported. A case of papillary adenocarcinoma of thymic origin that coexisted with type AB thymoma as a separate nodule is presented herein. The patient was found to have an abnormal mediastinal shadow on chest X-ray. A computed tomography scan revealed a round, 6.5-cm-diameter mass in the right anterior mediastinum. The preoperative diagnosis was thymoma, and thymothymectomy was performed. On pathological examination, two tumors, which were diagnosed as papillary adenocarcinoma and type AB thymoma, respectively, were present in the thymus without any connection with each other. The patient has been alive without any signs of recurrence for 11 years after surgery. We diagnosed the adenocarcinoma in this case was a primary thymic carcinoma. © 2010 The Japanese Association for Thoracic Surgery.