Sacrococcygeal teratoma (SCT) is a neoplasm derived from more than one primitive germ cell layer. SCTs have rarely been reported in adults. We present a case of a 17-year-old youth who complained of a painful mass over his left buttock. The mass had been growing gradually since his childhood. Magnetic resonance imaging showed a well-defined soft-tissue mass which contained mainly fat tissue and large intestine. The patient had complete excision of the tumor and coccyx. The gross impression and histology revealed a mature teratoma.