Abstract. Ovarian malignancies are rare in pregnancy; however, the incidence of abnormal adnexal masses diagnosed during pregnancy is increasing. The most common masses are ovarian cysts, and only 3-6% of those are malignant. The majority of ovarian masses are diagnosed at an early stage by routine ultrasound examinations. Malignant germ cell tumors are the most common ovarian malignancies associated with pregnancy, while the incidence of epithelial ovarian cancer is only 1:12,000-1:50,000 of pregnancies. The diagnosis and management of ovarian cancer during pregnancy remain unclear due to the rare occurrence and scant data on this condition. We herein report the case of 23-year-old woman with an extremely large ovarian papillary mucinous cystadenocarcinoma diagnosed during pregnancy, identified on ultrasound and magnetic resonance imaging, and treated by surgical resection followed by adjuvant chemotherapy with carboplatin and paclitaxel.