Endometriosis is one of the most common benign gynecological diseases. It shows similar attributes to those of some fatal tumors if it is becomes malignant. These attributes include invasion, implantation, and recurrence. Epidemiological, clinicopathological, molecular biological, and genetic evidence suggest that malignancy of endometriosis, referred to as endometriosis-associated malignancy (EAM), is histologically closely related to endometriosis. Atypical endometriosis, which usually causes EAM, is considered a transitional condition from benign endometriosis to cancer. Approximately 80% of EAMs occur in the ovary and are known as endometriosis-associated ovarian cancer (EAOC). However, extragonadal endometriosis is not common, and no earlier study reported an association between malignant transformation of rectal endometriosis and high-grade serous cancer. We report a rare case of high-grade serous carcinoma resulting from rectal endometriosis and complicated with ovarian cancer. A 63-year-old Chinese woman was admitted with a complaint of abdominal distension. We diagnosed the patient with ovarian carcinoma and decided on elective cytoreductive surgery as treatment for the patient. During the surgery, we found a solid mass of diameter 12 cm in the anterior rectal wall containing sticky brown fluid. Postoperative histopathological examination revealed high-grade serous carcinoma resulting from rectal endometriosis and complicated with ovarian cancer. The patient postoperatively received 6 cycles of chemotherapy consisting of carboplatin and paclitaxel and was followed up for 1 year with no recurrence of the condition.