Cystic adenomyosis is a rare form of adenomyosis of the uterine myometrium that has been described in older adults. This condition has not previously been reported in the pediatric radiology literature. In this case report, we describe an adenomyotic cyst in an adolescent girl that was imaged with US, CT, and MR and proved by surgical pathology. Understanding of the radiologic features of cystic adenomyosis is important for noninvasive diagnosis and effective medical/surgical management.