Abstract Malignant transformation of benign cystic teratoma of the ovary is rare, with an incidence of 1.8%. The commonest malignant neoplasm to develop is squamous carcinoma (80%). Adenocarcinoma occurs with less frequency, and only one of which has ever been cited to be of gastrointestinal origin. A 38-year-old female underwent TAH-BSO due to a large right ovarian tumor. Microscopically and immunohistochemically, the tumor was defined as mucinous adenocarcinoma originating from gastrointestinal epithelium in benign cystic teratoma.