Anaplastic thyroid cancers are the final dedifferentiation form of follicular thyroid tumors. They are one of the most serious cancers with a survival average of 6 months after diagnosis. Many studies suggest that anaplastic thyroid cancers are a form of a pre-existing differentiated thyroid carcinoma, but only a few cases have been reported in the literature. We report the case of a 64-year-old patient, with a 30-year history of a thyroid nodule. He presented a latero cervical swelling with compression signs, evolving for 2 months in a context of general state alteration. The tomodensitometry objectified a malignant process of the right thyroid compartment with repression of the upper airways. This process is associated with a large necrotic right jugulocarotid adenopathy, multiple lymph nodes, and secondary pulmonary parenchymal lesions (balloon release). The patient underwent a total thyroidectomy with lymph node dissection. The anatomopathological study revealed an anaplastic carcinoma of the thyroid developed on a papillary thyroid carcinoma (PTC). Thus supporting anaplastic transformation of PTC. Subsequently, the patient received radiochemotherapy but died from the disease 3 months after diagnosis. The case reports highlight that although rare, the transformation of a differentiated thyroid carinoma into anaplastic carcinoma can occur on the initial lesion or on metastases. This transformation radically changes the patients prognosis. Early management and rigorous long-term follow-up are therefore necessary.