The incidence of thymic cystic lesion is rare and its occurrence from ectopic thymic tissue is rarer. We report a case of multilocular thymic cyst from an ectopic superior mediastinal thymic tissue. This 24 years young male had diagnosis of a mediastinal mass and was referred to our institute for further management. A contrast computed tomography study of chest showed a multicystic superior mediastinal mass behind the superior vanacava and innominate artery. The surgical approach was midline sternotomy and a large 20x15x20 cm mass was excised. After one year of followup he has been doing well without fresh complaints.