Thyroid carcinoma showing thymus-like differentiation (CASTLE) is thought to originate from ectopic thymic tissue or remnants of the developing thymus within or adjacent to the thyroid. This case report describes a mass located on the left thyroid of a 28-year-old man. Fine-needle aspiration cytology revealed a number of lymphoid cells without atypia that were similar to those seen in a malignant lymphoma of the thyroid, and surgery was performed. Based on additional histopathological findings, the tumor was finally diagnosed as a CASTLE. It is difficult to diagnose this neoplasm using fine-needle aspiration cytology. However, it is possible to differentially diagnose CASTLE based on its histological features. CD5 is useful for diagnosing CASTLE with immunohistochemical staining.