The patient is a 74-year-old female previously diagnosed with an ovarian tumor at age 55. No changes were noted for one year; however, she was lost to follow-up. Eighteen years later, she presented to a local clinic complaining of diffuse abdominal and flank pain. Abdominal and pelvic ultrasound, CT, and gynecological examination showed a fatty pelvic tumor of approximately 12 cm in diameter. A left ovarian teratoma was suspected, and per the patient's request, she was transferred to Kobe Adventist Hospital for further evaluation and treatment. Pelvic MRI revealed no ovarian enlargement; however, a mass in the uterine body was appreciated with a high signal on T1 and T2 images and signal dropout in the fat suppression images, a finding most consistent with a uterine lipoma. A total hysterectomy and bilateral salpingo-oophorectomy was performed, and histopathological examination confirmed the preliminary diagnosis. No complications were observed during the postoperative period. A pure uterine lipoma is an extremely rare tumor with only a few cases reported worldwide. It is a benign tumor; however, it can sometimes be misdiagnosed as a malignant neoplasm. Pelvic MRI appears to be a useful tool in order to make the correct diagnosis preoperatively.