Increasing number of uterine malignancies have been reported in breast cancer patients using tamoxifen. Most of these are endometrial adenocarcinomas. However, only a few cases of endometrial stromal sarcomas have been reported to be linked with tamoxifen usage. A 58-year-old postmenopausal women who had been using tamoxifen for 4 years after a surgery for breast cancer is presented with chronic pelvic pain. Preoperative investigations were indicative of a uterine myoma so that a standard total abdominal hysterectomy and bilateral salpingo-oophorectomy were performed. Postoperative histologic diagnosis was a uterine tumor resembling ovarian sex cord tumors, which is an exceedingly rare entity itself. The present case is the first designated diagnosis of this rare tumor, with a possible association of tamoxifen usage.