Mediastinal ectopic thyroid is a very rare condition, with few reported cases in the literature and no reported cases in Korea. This report describes an asymptomatic 65-year-old man with a right paratracheal mass compressing the superior vena. Additionally, the epidemiology, clinical manifestation, diagnosis, and management of mediastinal ectopic thyroids are discussed. A mediastinal ectopic thyroid should be considered in the differential diagnosis of all mediastinal masses. Surgical excision is recommended for both the diagnosis and treatment of this condition, because of its potential for malignancy and compression of mediastinal structures. This case demonstrates the clinical importance of mediastinal etopic thyroid.