Thyroglossal duct cysts (TGDC) are the most common congenital cysts in the neck. TGDC typically present as a midline swelling anywhere between the foramen cecum and the isthmus of the thyroid gland. Majority occurs just below the hyoid bone and those descending below the thyrohyoid membrane are rare. TGDC are usually less than 3 cm in diameter while big cysts are uncommon. We present a 19 years old female patient who presented with giant (12 cm diameter) supra-sternal notch cyst which was diagnosed as TGDC only after histopathology. The rarity of such huge TGDC in such uncommon site, which was misdiagnosed as a retention cyst (Mucocele) made us report this case.