Low-grade endometrial stromal sarcoma (ESS) is a rare neoplasm that is generally estrogen-receptor- and progesterone-receptor-positive and develops in premenopausal women. Although tamoxifen treatment is associated with an increased risk of ESS, the effect of other selective estrogen receptor modulators, including toremifene, on the risk of ESS is not clear. A 61-year-old postmenopausal woman was treated with toremifene as an adjuvant therapy for breast cancer. A cystic mass developed during the treatment, with gradual growth in the uterine myometrium. The patient was treated with hysterectomy and bilateral salpingo-oophorectomy, and the tumor was diagnosed as low-grade ESS (stage IA) with estrogen-receptor and progesterone-receptor. The patient discontinued toremifene and has been progression-free for 21 months. Our data suggest that toremifene might be associated with the development of ESS in certain patients through its estrogen-like effects in the uterus.