Mature cystic teratoma is the most common type of ovarian germ cell tumor. Malignant transformation is a rare complication in 1-3% of cases, especially in post-menopausal women. The most common type of malignant transformation by histology is squamous cell carcinoma, followed by adenocarcinoma, carcinoid tumor, melanoma, and sarcoma. Diagnostic difficulties exist due to non-specific findings. No clinical, radiological, or biological signs are specific to malignant transformation. Most patients are diagnosed in advanced stages and have poorer outcomes. Staging of the disease is an important prognostic factor, with early diagnosis and treatment being critical for improved survival. Here, we report a rare case of mature cystic teratoma presented as torsion, postoperatively diagnosed as mature cystic teratoma with borderline mucinous cystadenoma and focal intra-epithelial malignant transformation.