Mature cystic teratoma (MCT) is the most common germ cell tumor of the ovary, comprising 20% of all ovarian neoplasms. Malignant transformation (MT) is an uncommon complication and occurs in approximately 1–3% of all MCTs. The most common histological type of MT is squamous cell carcinoma. Other rare transformations include – carcinoid tumors, melanoma, adenocarcinoma, and sarcoma. We present one such rare case of bilateral MCT with unilateral MT into mucinous adenocarcinoma of intestinal type.