Our case involves a 68-year-old postmenopausal patient with a history of total abdominal hysterectomy and right salpingo-oophorectomy performed 25 years ago. The patient presented with chronic pelvic pain for a gynecological examination. Clinically, a large painless pelvic mass was palpable, likely originating from the preserved left ovary. Transvaginal ultrasonography, computed tomography, and magnetic resonance imaging supported the clinical suspicion, and an exploratory laparotomy was subsequently performed. Intraoperatively, a solid, neoplastic, exophytic ovarian tumor was discovered, occupying the entire pelvis. The tumor, along with the ovary and the corresponding fallopian tube, was surgically removed. Histological examination of the specimen confirmed the diagnosis of an ovarian thecoma. Following a four-day hospitalization with an uneventful postoperative course, the patient was discharged from our clinic. Three months later, she reported complete relief from chronic pelvic pain. This report of the case also includes a brief literature review, emphasizing the preoperative diagnostic challenges of this rare clinical entity and the importance of regular gynecological check-ups for women who have undergone total hysterectomy with ovarian preservation.