This article reports the diagnosis and treatment process of a patient with bladder diverticulum carcinoma. The patient presented to a local hospital in May 2023 with recurrent gross hematuria. Imaging studies suggested a mass in the diverticulum of the right bladder wall, and cystoscopic biopsy pathology revealed high-grade non-invasive papillary urothelial carcinoma. The patient underwent open partial cystectomy. Postoperative pathology indicated muscle-invasive urothelial carcinoma, but no further treatment was received initially. Subsequently, the patient visited our hospital, repeated contrast-enhanced CT, MRI, and cystoscopy showed no definite evidence of residual tumor. However, based on literature review, perioperative data, and the possibility of extravesical extension not being ruled out, radical cystectomy was recommended. But the patient strongly desired bladder preservation and therefore received three cycles of immunotherapy with toripalimab. Follow-up examination in September of the same year revealed thickening and enhancement of the right bladder wall on routine imaging. A multidisciplinary consultation on difficult cases in Guangdong Urological Association was conducted, the experts suggested that based on the comprehensive examination results, residual tumor and lymph node metastasis could not be excluded. The diagnosis was high-grade bladder diverticulum carcinoma, which is highly malignant and may involve early invasion of the bladder wall layers. The case did not meet the recommended criteria for bladder-preserving therapy, and there were signs of progression even after immunotherapy. Therefore, it was recommended to perform a PET-CT scan and proceed with radical cystectomy as soon as possible. Subsequent treatment (including radiotherapy, chemotherapy, or immunotherapy) would depend on the postoperative pathology, and close follow-up was necessary. After careful consideration, the patient returned to the hospital and underwent robot-assisted laparoscopic radical cystectomy. Intraoperatively, a frozen pelvis was found on the right side, and frozen section pathology confirmed metastasis to the right pelvic lymph nodes and invasion of the pelvic wall. An ileal conduit urinary diversion was subsequently performed. Postoperative immunohistochemical pathology confirmed regional urothelial carcinoma in situ of the bladder, with metastasis to the pelvic lymph nodes and pelvic wall. This case suggests that postoperative recurrence in bladder diverticulum carcinoma may not be local and could involve distant metastasis. Bladder-preserving therapy should be reserved for selected, suitable patients with favorable profiles, and should not be pursued blindly.