Teratomas in the neck are rare neoplasms. Most occur in the neonate and are benign. In contrast, cervical teratomas in adults are malignant and carry a poor prognosis. Fourteen adult cases have been reported in the world literature, and 11 of these were stated to have arisen in the thyroid gland. A case of a 27-year-old man who presented with a large goiter is reported. A preoperative diagnosis of a malignant thyroid mass was suggested by clinical examination, needle aspiration, and computerized tomography. The patient had a subtotal debulking thyroidectomy. The pathologic specimen confirmed a primary malignant teratoma of the thyroid gland. Despite a combination of radiotherapy and chemotherapy, which were well tolerated, the patient died 2 months after surgery.