INTRODUCTION Type AB thymoma is generally considered a benign tumor with excellent prognosis. Rare case reports of type A and AB thymomas with recurrence and metastasis have been documented, although extrathoracic metastases are extremely rare. To the best of our knowledge, this is the first reported case of type AB thymoma with solitary metastasis to the breast. CASE PRESENTATION We describe an 83-year-old Taiwanese woman with a metastatic thymoma to the breast 10 years after complete resection of noninvasive and encapsulated primary tumor, and analyze the possible factors to explain the recurrence and metastasis of the stage I thymomas. CONCLUSIONS Even a clinically benign tumor such as type AB thymoma still has a possibility of metastasizing to an unusual site. When any uncommon tumor presents in any site, a suspicion of secondary neoplasm and thorough clinical history are required.