Ectopic thyroid cancer is extremely rare disease, only a few case reports are published about it. Ectopic thyroid cancer usually arises from ectopic thyroid tissue. The predominant location of ectopic thyroid tissue is the lingual thyroid, whereas a mediastinal thyroid is uncommon. Ectopic thyroid cancer below the neck is extremely rare; therefore, the diagnosis of ectopic thyroid cancer below the neck is based on pathologic results. We describe here a case of asymptomatic mediastinal ectopic thyroid cancer with a normal thyroid gland, underwent mediastinal mass excision and no relapse for 3 years.