Background <p>With the rising rate of cesarean section delivery worldwide, the development of cesarean scar diverticulum (CSD) is increasingly observed, with a prevenance ranging from 24% to 70%. CSD is associated with abnormal uterine bleeding, chronic pelvic pain, cesarean scar pregnancy, and infertility. Furthermore, a CSD can present as a pelvic mass that causes diagnostic dilemma. Herein, we report a rare case of uterine cystic adenomyoma (UCA) arising from CSD and review a series of similar diseases to improve the understanding of CSD-associated mass.</p> Case presentation <p>A 37-year-old woman presented with menorrhagia after her two cesarean deliveries. Transvaginal sonography (TVS) revealed a huge, thick-wall, heterogeneously hypoechoic mass at the anterior lower uterine segment, which was connected to uterine cavity and protruded outside of the uterus. No specific ultrasound diagnosis was made although it was in favor of benign lesion from prior cesarean scar. Hysteroscopic and laparoscopic exploration was decided. The operation verified a CSD at previous cesarean scar. Postoperative pathology revealed a CSD, uterine cystic adenomyoma (UCA), and diffuse smooth muscular hyperplasia, in favor of the diagnosis of CSD accompanied by UCA. The patient’s menstrual condition improved after surgery.</p> Conclusions <p>CSD-associated mass is very rare. In patients with a prior cesarean section, a mass in the lower uterine segment, especially with a cavity connection, should raise suspicion for a CSD-associated lesion. When TVS is inclusive, pelvic MRI serves as a diagnostic adjunct. For management, the combined hysteroscopic-laparoscopic approach allows for precise excision of the mass and concurrent repair of the CSD in a single procedure. Future efforts should focus on refining the classification of these masses and standardizing management procedures to optimize symptomatic and reproductive outcomes.</p>