The authors present a case of ectopic cervical thymus in a 4-month-old child. Ectopic cervical thymus is an uncommon etiology of a neck mass in a child. It is often misdiagnosed preoperatively as a possible malignancy. It is usually located along the embryologic course of thymus migration from the angle of the mandible to the superior mediastinum. Its treatment is surgical since malignant transformation and respiratory infection have been described. No case of immune deficit after total thymectomy has been described in the literature.