Objective: Ovarian mature cystic teratomas with malignant transformation are rare, and squamous cell carcinoma is the most common pathological entity. Among these malignant transformations, urothelial cell carcinoma is rare. Case Report: We report a woman presenting with a huge pelvic cystic mass, favoring a right ovarian mature cystic teratoma with malignant transformation, based on magnetic resonance imaging, who was successfully treated with surgery. Conclusion: The final pathology confirmed concomitant malignant transformation of urothelial carcinoma.