Intrathyroidal thymoma is a rare tumor that occurs in the thyroid gland. The present report describes a 62-year-old female patient with this tumor, who was treated with subtotal thyroidectomy with a wedge resection of the trachea followed by radiotherapy. She is now free from disease for 4 years after the operation. This tumor shows expansive growth, frequent invasion to the surrounding tissue, but relatively infrequent lymph node involvement. If this tumor is resected surgically, the prognosis is favorable.