Tamoxifen has been widely used for adjuvant treatment of breast cancer, but several gynecological side effects have been noted, including endometrial hyperplasia, polyp and carcinoma. Polypoid endometriosis is one of the extremely rare benign complications associated with tamoxifen therapy. A 66-year-old postmenopausal woman, who had received left partial mastectomy due to breast cancer (about 4 years ago) and was taking tamoxifen treatment, had an ovarian cyst on ultrasonography. Pelvic magnetic resonance imaging suggested tamoxifen-associated endometrial and ovarian changes, especially a 4.1×3.4-cm-sized, well-defined, multicystic mass in the right ovary. She received hysterectomy with bilateral salpingo-oophorectomy. Microscopically, the right paratubal mass showed endometrial glands and stroma, and immunohistochemical staining for CD10 confirmed the endometrial nature of the stroma. Three cases of polypoid endometriosis have been reported in the Korean literature, but in none of the cases, polypoid endometriosis was associated with tamoxifen use. Herein, we report the first case of polypoid endometriosis associated with tamoxifen treatment in Korea.