We report a case of metastatic breast tumor arising from a synovial sarcoma of the lower limb. A 27-year-old Japanese woman was diagnosed to have synovial sarcoma 14 months prior to finding a mass in her left breast. An excisional biopsy was performed and a metastatic synovial sarcoma to the breast was confirmed. Eight months after the resection of the breast lesion, the patient developed local recurrences in both her knee and breast. A tumor resection of the limb lesion and a simple mastectomy for the huge lesion, which was diagnosed to be a metastatic breast tumor without lymph node metastasis, were performed. After the operation, the patient received adjuvant systemic chemotherapy. To the best of our knowledge, this is only the second reported case of a solitary metastatic breast tumor arising from synovial sarcoma.