We report a case of extramammary Paget's disease in ovarian mature cystic teratoma. The patient was a 70-year-old Japanese woman who complained of lower abdominal pain. Examination showed elevation of carcinoembryonic antigen and CA 19-9. Ultrasonography and computer tomography revealed a cystic tumor of the left ovary. The gross appearance of the resected ovary was typical for mature cystic teratoma. Microscopic observation revealed a lesion of Paget's disease within the squamous epithelium. The tumor cells had intracytoplasmic mucin and positive immunoreactivity for carcinoembryonic antigen, epithelial membrane antigen, and cytokeratin; but they were negative for S-100 protein and vimentin. On multiple and serial sections, underlying adenocarcinomas were not found either in the ovary or other primary sites. From these pathological findings, we concluded that the disease was an intraepithelial adenocarcinoma, possibly derived from multipotential cells in squamous epithelium of ovarian mature cystic teratoma. This is the first reported case, to our knowledge, of extramammary Paget's disease arising in mature cystic teratoma of the ovary.