Mature cystic teratoma is the most common type of ovarian germ cell neoplasm, but occasionally, it can undergo malignant transformations, especially in postmenopausal women. These secondary malignant neoplasms are most commonly squamous cell carcinomas. The absence of clinical and radiological specificity of this transformation means that the diagnosis remains purely histological. Data is insufficient regarding the appropriate management given their rarity. However, the treatment is multidisciplinary and is based on surgery and a platinum-based chemotherapy regimen. We report the case of a 53-year-old postmenopausal female patient with malignant transformation of the ovarian teratoma who was treated surgically and whose outcome was favorable. The diagnosis of the teratoma was evoked on imaging, while the diagnosis of squamous cell carcinoma was revealed on histology. Malignant transformation is an uncommon complication of mature ovarian teratomas. No clinical, radiological, or biological sign is specific; therefore, resection of any ovarian mass, even asymptomatic, is required.