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Malignant transformation of a mature cystic teratoma (MCT) is rare, with mucinous adenocarcinoma being an even more uncommon subtype. Accurate diagnosis requires distinction from metastatic gastrointestinal tumors. A woman in her 30s presented with a 19 cm right ovarian mass. She underwent exploratory laparotomy with right salpingo-oophorectomy, left ovarian cystectomy, omentectomy, peritoneal biopsies, and bilateral pelvic and para-aortic lymphadenectomy with intraoperative rupture of the mass. Frozen section suggested immature teratoma, but final pathology revealed mucinous adenocarcinoma arising within a mature teratoma. The tumor showed CK20, CDX2, SATB2 positivity, patchy CK7, and loss of PAX8. All lymph nodes and staging biopsies were negative for carcinoma. Final stage: FIGO IC1. This case highlights the diagnostic and staging challenges in malignant transformation (MT) of a MCT. Thorough sampling, immunohistochemistry (IHC), and clinical correlation are critical to differentiate primary ovarian neoplasms from metastatic disease. Accurate classification is essential to guide appropriate management.