The fibrous variant of Hashimoto thyroiditis is uncommon, accounting for approximately 10% of all cases of Hashimoto thyroiditis. We report a case of this variant that behaved like a malignant neoplasm. The patient was a 69-year-old man who presented with a right-sided anterior neck mass that had been rapidly growing for 2 weeks. Fine-needle aspiration cytology revealed clusters of large multinucleated cells suggestive of an anaplastic carcinoma. A week after presentation, we ruled out that possibility when the mass had shrunk slightly. Instead, we diagnosed the patient with an acute exacerbation of Hashimoto thyroiditis on the basis of laboratory findings. We performed a right thyroid lobectomy, including removal of the isthmus, to clarify the pathology and alleviate pressure symptoms. The final diagnosis was the fibrous variant of Hashimoto thyroiditis, with no evidence of malignant changes. Physicians should keep in mind that on rare occasions, Hashimoto thyroiditis mimics a malignant neoplasm.