Cowden syndrome is a rare autosomal dominant disorder characterized by multiple hamartomas in various tissues, including the skin, mucous membranes, and gastrointestinal tract. Germline mutations of the PTEN tumor suppressor gene are responsible for Cowden syndrome. Cowden syndrome is associated with an increased risk of breast, thyroid, renal and uterine cancers; however, ovarian cancer rarely develops in women with Cowden syndrome, although somatic PTEN mutation often occurs in some types of ovarian carcinomas. Herein we report the first case of ovarian carcinosarcoma that developed in a woman with Cowden syndrome. A 55-year-old woman with a history of breast cancer, thyroid goiter, and palatal papillomatosis presented with pelvic distention. CT scan revealed a pelvic tumor suggesting ovarian cancer. She underwent a total abdominal hysterectomy, a bilateral salpingo-oophorectomy, and an omentectomy, but the surgical cytoreduction was suboptima( l >2 cm residual disease). Pathological examination showed a mixed tumor composed of high-grade carcinoma and heterologous sarcoma. Immunohistochemically, tumor cells were positive for p53. She was diagnosed with stage ⅢC ovarian carcinosarcoma. Genetic testing detected a PTEN variant, confirming the diagnosis of Cowden syndrome. She received paclitaxel/ carboplatin chemotherapy. However, no response was observed and she died of disease 2 months postoperatively.