An extensive review of the literature suggests that ours is the first case of encapsulated thymoma metastasizing to a skeletal muscle. A 43-year-old man underwent thymothymectomy for encapsulated Masaoka's stage I thymoma. Four years after complete resection, the tumor metastasized to the left pectoralis major muscle. Although a few reports exist on encapsulated thymoma metastasizing to a distant site, the literature does not describe encapsulated thymoma metastasizing to a skeletal muscle insofar as we could find.