Malignant transformation is a rare complication of mature cystic teratoma with squamous cell carcinoma being the most common type. We report a new case of squamous cell carcinoma in situ. A 62 year old woman was admitted for an abdomino-pelvic mass and she underwent a left salpingo-oophorectomy. The histopathologic analysis revealed a squamous cell carcinoma in situ arising in mature cystic teratoma of the ovary. Then, she underwent a total hysterectomy, contralateral salpingo-oophorectomy and omentectomy without adjuvant treatment. Optimal cytoreduction has been associated with a statistically significant improvement in survival for malignant transformation of mature cystic teratoma.