A postmenopausal woman, in her 60s, married with no history of pregnancy, presented to our facility with an abdominal mass of 2 months duration that progressively increased in size. Physical examination revealed a mobile abdominal cystic mass measuring approximately 20×20×10 cm, accompanied by ascites. Transvaginal ultrasonography showed a hypoechoic cystic mass measured >
14.6×12.5 x13.5×11.49 cm, with septa and papillary projections, and the Doppler colour score was +2, along with ascites. Based on the IOTA Simple Rules, this mass would be classified as malignant, because it meets the criteria for M2 (ascites) and M4 (irregular multilocular solid tumour ≥100 mm) features. Because malignancy was suspected, a total hysterectomy and bilateral salpingo-oophorectomy with a frozen section were performed. The frozen section and histopathological examination resulted in a right endometrioma. Postmenopausal endometriosis is rare, but clinicians should always be aware of its existence as it confers a risk of malignant transformation and recurrence. Comprehensive follow-up after surgery is recommended in such cases.