We report a case of ectopic cervical thymoma that was difficult to differentiate from thyroid tumor. A 69-year-old woman was referred to our hospital with a tumor on the left side of the neck. Fine-needle aspiration cytology could not establish the diagnosis and the surgery was then performed for diagnosis and treatment. Since the intraoperative pathological diagnosis was also inconclusive, thymectomy was performed because of the macroscopic finding suggesting close relation to the thymus. The final pathological diagnosis was type AB thymoma.