We report an unusual case of adenocarcinomatous transformation of a paracoccygeal teratoma (PCT) in a 44-year-old woman. The patient was referred to us for surgical treatment of a growing paracoccygeal mass. The findings of a magnetic resonance imaging scan were compatible with soft tissue malignancy. Using the posterior approach, we performed complete surgical removal with coccygectomy. Histological examination of the cystic mass revealed a mature teratoma containing primary adenocarcinoma. The patient has been followed up for 7 years and is free of disease. Because prognosis depends on complete excision, we emphasize the importance of clear-margin surgery, including en bloc removal of the coccyx.