A case of adenomyomatous polyp of the uterine endometrium is reported. The patient was 64-year-old woman treated with tamoxifen. Ultrasonography demonstrated a heterogeneous hyperechoic mass with small cystic spaces in the uterus. On magnetic resonance imaging (MRI), a thick stalk with many thin restiform branches within a large solid and cystic endometrial mass showed dendriform low intensity on T2-weighted images and intense enhancement on contrast-enhanced T1-weighted images. The prominent dendriform central fibrous core on MRI reflected a thick stalk originating from the myometrium with numerous branches containing abundant smooth muscle fibers, and may represent the pathologic feature of adenomyomatous polyp.