Branchial cleft fistulae are rare congenital anomalies that arise from the abnormal persistence of branchial remnants. Branchial arch anomalies are rare. They usually present as a lateral neck mass or abscess in the form of acute suppurative thyroiditis. A complete fistula of the third arch is extremely rare. We describe such a case in a 13-year-old girl who presented with a small opening in the left lower neck, from which a mucoid discharge had been present since birth. The fistula was accompanied by recurrent neck swelling. Computed tomography with contrast injection into the external skin opening revealed a continuous tract that extended to the base of the piriform sinus. Total excision of the tract up to the piriform sinus with a left hemithyroidectomy was performed. At follow-up 28 months postoperatively, the patient exhibited no evidence of recurrence.