INTRODUTION: Carcinoma showing thymus-like differentiation (CASTLE) is a rare malignant thyroid cancer. We report a case of CASTLE treated with surgery and adjuvant external neck radiation therapy. MATERIALS AND METHODS: We experienced a case of CASTLE in a 63-year-old man, who presented with a neck mass due to enlarged and hard thyroid gland, most notably in the right lobe, without palpable cervical nodes. RESULTS: Total thyroidectomy was performed for both diagnostic and therapeutic purposes. Histologic examination of the specimen showed a “thymus like” thyroid cancer; the tumor, partially circumscribed, lobulated, involved both lobes, and in more points reached the capsule of the organ and infiltrated perithyroid muscle tissue. After immunohistochemical assay (CK19 +, CK20 -, thyroglobulin -, calcitonin -, CD5 +, CD117 +), a diagnosis of CASTLE was made. DISCUSSION: CASTLE is a rare, malignant tumour of the thyroid gland, with histopathological features similar to squamous cell carcinoma, but with a more favourable prognosis. Preoperative diagnosis is difficult. CASTLE is usually cured surgically with total thyroidectomy and selective neck dissection, followed by radiotherapy. CONCLUSIONS: Surgery and radiotherapy are effective to manage thyroid CASTLE tumors.