Background <p>Bladder diverticulum is a rare pelvic mass that arises from a defect between the detrusor muscle fibers. It does not often cause distinct symptoms and is most commonly detected accidentally in postmenopausal women as an ovarian cyst while investigating unrelated ailments. Therefore, preoperative diagnosis is difficult, and there are cases of intraoperative embarrassment in the absence of an ovarian cyst.</p> Case presentation <p>A 76-year-old woman who was diagnosed with an ovarian cyst using abdominal computed tomography (CT) at another institution was referred to our department for further evaluation. Magnetic resonance imaging (MRI) showed similar findings, and the radiologist diagnosed it as a serous ovarian cyst, for which laparoscopic surgery was planned. Laparoscopic findings however, did not reveal any evidence of the preoperatively diagnosed ovarian cyst in the pelvic cavity. Furthermore, due to swelling in the left retroperitoneal cavity, the area was further explored. A mass with serous content was identified and resected. During the procedure, the ureteral balloon catheter was exposed, revealing that the excised mass was a bladder diverticulum complicated by bladder injury. Subsequently, the bladder injury site was laparoscopically sutured and repaired. Four weeks post-surgery, urological imaging confirmed complete reconstruction of the bladder injury, and the indwelling ureteral balloon catheter was successfully removed.</p> Conclusion <p>We experienced a case of complete laparoscopic excision and bladder injury repair of a massive bladder diverticulum mimicking an ovarian cyst. Bladder diverticula, though rare, should be considered in the differential diagnosis of pelvic masses, particularly in post-menopausal women with equivocal preoperative diagnoses to avoid unnecessary surgical interventions.</p> Precis <p>Bladder diverticula should be considered in the differential diagnosis of pelvic masses, particularly in postmenopausal women, as they can mimic ovarian cysts.</p>