We report a case of a cystic adenomyoma in a 19-year-old nullipara who presented with severe dysmenorrhea and pelvic pain. Preoperative magnetic resonance imaging showed a 3-cm well-circumscribed mass with a 2.1-cm cystic cavity in the myometrium of the left fundus. The tumor, including the cyst, was excised via laparoscopy with a part of the normal myometrium using a modified myomectomy method. On histologic examination, the cystic cavity was lined by endometrial tissue composed of endometrial epithelium and stroma. After 12 months of postoperative follow-up, the patient had significant improvement of dysmenorrhea.