BACKGROUND: Metastasis of extragenital neoplasms to an endometrial polyp is rare and until now, only 6 cases of such involvement has been described. CASE: A 58-year-old woman, who had been diagnosed 4 years ago with infiltrating ductal breast carcinoma and treated with surgery and tamoxifen therapy, was admitted to the gynecology clinic because of endometrial thickening observed during a routine abdominal ultrasonographic examination. A total hysterectomy with bilateral salpingo-oophorectomy was performed. Pathological examination of the specimen showed a large polyp which microscopically showed clusters of cells with signet ring morphology within the polyp stroma. The positivity of tumor cells for GCDFP-15 supported the diagnosis of metastatic breast carcinoma to endometrial polyp. CONCLUSION: Metastatic breast carcinoma should be considered in the differential diagnosis of carcinomas with signet ring cell morphology involving uterus.