Spindle Cell Carcinoma of the Ovary is arate form of cancer with controversial histogenesis. It shares the histologic, cytologic, and molecular properties of both epithelial and mesenchymal differentiation of ovarian neoplasms, which makes diagnosis very challenging among pathologists. Endometrioid cystadenoma is a benign ovarian neoplasms classified under epithelial ovarian tumors.Malignant transformation of benign ovarian neoplasms is known as a rare complication, occurring in approximately 0.9% of patients with ovarian endometriosis. Clear cell adenocarcinoma is the most common endometriosis-associated ovarian cancer followed by endometrioid cancer. This is the case of a 56-year old post-menopausal patient initially presenting with increasingabdominal girth. Whole abdominal ultrasound revealed a large pelvo-abdominal mass. Transvaginal and transabdominal ultrasound findings of bilateral ovarian new growth with benign sonologic features. The patient underwent bilateral salpingo-oophorectomy. Histopathologic findings of the specimen submitted revealed high-grade spindle cell carcinoma arising in an endometrioid cystadenoma of the right ovary, and endometrioid cystadenofibroma with focal epithelial proliferation.