Summary: A 25-year-old female complained of a mass on the left supraclavicular fossa for 4 months without any associated symptoms. Computed tomography and magnetic resonance imaging showed a thin-walled monocystic lesion just above the clavicle and dorsal-lateral to the sternocleidomastoid muscle. The aspirated fluid from the cyst showed a milky-white appearance and characteristic composition to coyle such as high concentration of triglyceride. Therefore, the cyst was excised with the preoperative diagnosis of a thoracic duct cyst. Intraoperatively, a branch from the thoracic duct entering the dorsal-medial wall of the cyst was found. The branch was ligated before complete removal of the cyst. Histological findings showed that the cyst wall contained an inner single layer of flat endothelial cells, smooth muscle cells and islets of lymphoreticular tissue.