Anaplastic thyroid cancer (ATC), a rare thyroid malignancy, accounts for only 5% of all thyroid cancers. However, it is the most aggressive form and has a very poor prognosis. Increasing evidence suggests that ATC arises from papillary thyroid carcinoma (PTC). However, the exact mechanism underlying this transformation remains unclear. In almost all cases, ATC originates within, but rarely outside, the thyroid gland. Transformation of metastatic PTC into ATC within the cervical lymph nodes is extremely rare. In this report, we present a rare case in a 63-year-old male patient who was initially diagnosed with PTC at his first hospital visit, which underwent anaplastic transformation in lymph node metastasis, and was subsequently diagnosed during the follow-up visit.