The common sites of breast cancer metastases include bones, lung, brain, and liver. Renal metastasis from the breast is rare. We report a case of breast cancer metastatic to the kidney with extension into the renal vein. A 40-year-old woman had undergone left mastectomy for breast cancer at the age of 38. A gastric tumor, which was later proved to be metastasis from breast cancer, was detected by endoscopy. Computed tomography performed for further examination of the gastric tumor revealed a large left renal tumor with extension into the left renal vein. It mimicked a primary renal tumor. Percutaneous biopsy of the renal tumor confirmed metastasis from breast cancer. Surgical intervention of the stomach and the kidney was avoided, and she was treated with systemic chemotherapy. Breast cancer metastatic to the kidney may present a solitary renal mass with extension into the renal vein, which mimics a primary renal tumor.