A 12-year-old male child was referred to our clinic for the surgical treatment of an anterior mediastinal mass, suspected to be a thymic cyst, which was considered to potentially lead to cardiac failure. The mass was resected completely with a median sternotomy. The postoperative course was uneventful. A pathological examination revealed a mature cystic teratoma of anterior mediastinum which is a very rare cystic tumor at this location. The incidence, diagnostic procedures, complications, and treatment of mediastinal teratomas are discussed along with a review of the literature.