We report a case of thyroid metastasis from breast carcinoma. A 41-year-old woman with a history of breast cancer was referred to our department for the treatment of a thyroid tumor with hoarseness. She had undergone mastectomy 2 years ago, with subsequent chemotherapy. During the 2-year interval, there had been no clinical recurrence of breast carcinoma. Total thyroidectomy was performed for the thyroid tumor. Pathological examination demonstrated metastatic breast carcinoma to the thyroid. The thyroid is usually considered to be a rare site of metastatic involvement from primary carcinoma of other organs. However, in a patient with a history of malignant tumor, the possibility of metastatic disease should be considered.