Primary thyroid fibrosarcoma cases are very rare. Although it is a known fact that soft tissue sarcomas show slow growth, there have been some cases in literature similar to our case in which there was a fast-growing tumour tissue causing breathing and swallowing difficulties due to painless pressure. For diagnosis, there is no specific clinical or radiological finding. We report a 67-year-old male with a mobile fast-growing mass covering almost all over the neck that appeared 2 months prior to the admission. Laboratory findings showed that the patient was euthyroid. Fine needle aspiration biopsy results are consistent with suspicion of a mesenchymal, histiocytic, epithelial or lymphoid tissue origined malignancy, Patient was taken into surgical operation. The thyroid tissue invaded the main vascular structure, trachea and esophagus. Due to this situation R1 resection was applied. lmmunohistopathological examination showed a conventional type of fibrosarcoma. After the surgery, radiotherapy and chemotherapy had been planned and applied. Patients died before the radiotherapy sessions ended. It should be kept in mind that a rapid growth in thyroid tissue can be thyroid fibrosarcoma, there could be a rapid clinical course and poor prognosis after operation.