A 57-year-old postmenopausal woman presented with synchronous tumors in both ovaries: a high-grade serous carcinoma in the right ovary and a benign mature cystic teratoma in the left. The patient presented with abdominal distension, discomfort and ascites, imaging revealed a massive right ovarian cystic mass with peritoneal involvement and a left adnexal lesion with fat and fluid levels suggestive of a dermoid cyst. CA-125 was markedly elevated and ascitic cytology confirmed malignant epithelial cells. She underwent hysterectomy with bilateral salpingo-oophorectomy and omentectomy, histopathology showed tubal and cervical stromal invasion in the right ovary. Immunohistochemistry confirmed Müllerian origin (WT-1+, PAX8+, p53 mutant pattern, high Ki-67) and the tumor was classified as FIGO IIIC. Postoperative platinum-based chemotherapy resulted in clinical improvement, reduction in CA-125, and no recurrence at 9 months. The case highlights the importance of correlation of imaging, cytology, extensive sampling, and immunohistochemistry to distinguish independent tumors.