A case of solitary fibrous tumor (SFT) of the thyroid in a 43-year-old woman with a multinodular goiter is reported. This is the first case of SFT described in the thyroid. On histologic, immunohistochemical, and ultrastructural examination, the tumor was identical to SFT of the pleura and other organs. Despite its rarity, SFT should be included in the differential diagnosis of spindle-cell tumors of the thyroid, along with anaplastic carcinoma, spindle-cell medullary carcinoma, and several types of mesenchymal tumors.