Synovial sarcoma (SVS) of the thyroid gland is exceedingly rare. We report the case of a 55-year-old man with a rapidly growing 7-cm neck mass. Because of suspicion of anaplastic thyroid carcinoma, a total thyroidectomy was planned, without preoperative cytology. During surgery, the tumor ruptured, leading to fragmented and incomplete resection. The morphological and immunohistochemical aspects suggested thyroid SVS, which was confirmed by fluorescent in situ hybridization (SYT gene rearrangement). The patient experienced immediate local relapse in close contact with large vessels and the thyroid cartilage and was referred to our institution. Doxorubicin-ifosfamide chemotherapy led to a minor response that authorized secondary conservative surgery. Because of microscopically incomplete resection, adjuvant radiotherapy was chosen and is ongoing 10 months after initial surgery. The prognosis of thyroid SVS is associated with a high risk for local and metastatic relapses. Pretreatment diagnosis is fundamental and may benefit from molecular analysis. Margin-free monobloc surgical excision is the best chance for cure, but adjuvant chemotherapy and radiotherapy deserve to be discussed.