A 52-year-old post-menopausal woman presented with abdominal distention and pain for 6 months. Imaging suggested ovarian cystadenocarcinoma, but surgery revealed a rare occurrence: a malignancy arising within a mature cystic teratoma (MCT). While MCTs are common ovarian neoplasms, malignant transformation is rare, occurring between the 5th to 7th decade of life. In this case, a poorly differentiated adenocarcinoma with colorectal differentiation emerged within the teratoma. Proper sampling and meticulous examination are crucial for diagnosis. Adenocarcinomas within MCTs often originate from gastrointestinal or respiratory epithelium and commonly exhibit TP53 mutations. This unique case showcased malignancy involving the endodermal component without recognizable benign endodermal constituents within the teratoma. Treatment involved cytoreductive surgery and adjuvant chemotherapy. Precise diagnosis demands a deep understanding of the entity, comprehensive gross sampling, and accurate radiological assessments to rule out metastasis. Immunohistochemical studies, clinical-radiological correlation, and tumor morphology are vital for accurate diagnosis and subsequent management. The patient, after treatment, showed no evidence of residual disease during follow-up.