Mediastinal tumors are a common condition encountered by general thoracic surgeons, and the most frequent of such tumors is thymoma. However, metastatic mediastinal tumors are rare. Here we report a case of papillary carcinoma of the thyroid to the mediastinum that mimicked thymoma. The patient, a 60-year-old woman who had been treated for thyroid cancer 6 years previously, visited our hospital because of an abnormal shadow in the anterior mediastinum. Radiological findings and laboratory data did not suggest any recurrence of the thyroid cancer, but computed tomography examination revealed a thymoma-like mass. (18)F-Fluoro-2-deoxy-D: -glucose positron emission tomography revealed two lesions: an anterior mediastinal mass and a right breast nodule. We resected both tumors. Histological examination revealed that the mediastinal mass was a metastasis from the thyroid papillary carcinoma, whereas the nodule was an early breast cancer. At the 9-month follow-up examination the patient was well without evidence of recurrence of either cancer.