Adenomyotic cysts are uncommon findings, usually in the context of diffuse adenomyosis and <5 mm in diameter. Herein we report a 4.5-cm adenomyotic cyst in a 25-year-old nulliparous woman with severe dysmenorrhea and pelvic pain. Transvaginal ultrasonography and magnetic resonance imaging revealed a well-circumscribed hypoechogenic mass in the posterior uterine wall, well separated from the uterine cavity. Pathologic analysis demonstrated that the cyst was lined with endometrial epithelium and stroma and was surrounded by smooth muscle hyperplasia. In the literature, we found 30 reports of cysts with similar characteristics. Because this cyst has not been clearly defined, it has been called by various names including adenomyotic cyst, cystic adenomyosis, and cystic adenomyoma. We believe this lesion should not be called an adenomyoma, but is more correctly called an adenomyotic cyst or, depending on age at onset, a juvenile adenomyotic cyst.