A malignant tumor is generally believed to be very unlikely to metastasize to the thymus. Only three cases of thymic metastases have been reported so far in the medical literature. We report here a rare case of metastatic breast cancer to the thymus, which was detected by CT and PET scanning, and the metastasis was also confirmed by video-assisted thoracic surgery biopsy. Recognition of an unusual breast cancer metastasis, such as to the thymus, as well as the usual patterns of breast cancer metastasis will facilitate an accurate, prompt diagnosis and its appropriate treatment.