A 58-year-old woman was pointed out an abnormal electrocardiogram during a physical examination. A chest computed tomography( CT) scan revealed a 50-mm-sized cystic mass in the posterior mediastinum near the tracheal bifurcation. The mass exhibited growth during follow-up imaging, leading to a surgical resection. The tumor appeared as a smooth mass lesion with milky white and turbid internal content, suggestive of a chyle. The presence of a contiguous lymphatic duct and positive CD31 staining on histopathological examination confirmed the diagnosis of a thoracic duct cyst. Thoracic duct cysts have the risk of rapid growth and rupture, so in addition to checking for the appearance of new subjective symptoms, we suggest that surgical treatment should be considered if a tendency toward enlargement is observed as part of a shorter-than-usual imaging follow up. Careful identification and management of the thoracic duct flowing into and out of the tumor are important to prevent postoperative complications, especially chylothorax.