The differential diagnosis of a lateral cystic neck mass includes branchial cleft cysts and lymphangiomas. Cervical thymic cysts are rare, and thymopharyngeal duct cysts, which maintain a connection to the pharynx, are rarer still. We present an interesting case of a 6 year-old male who developed acute onset of a left-sided neck mass. CT and MRI findings revealed a multiloculated cyst closely associated with the left lobe of the thyroid gland with a tract extending up along the carotid sheath to the pyriform sinus. Complete surgical excision was performed. The anatomic location and pathology were consistent with a thymopharyngeal duct cyst. The embryology, clinical presentation, radiologic and pathologic findings, and surgical management of thymopharyngeal duct cysts will be discussed.