Gangrenous cystitis is a pathology that is rarely encountered in current urological practice. It is due to necrosis of the bladder wall, which may be superficial or involve the entire wall. Its exact pathogenesis is unknown, but several factors make its diagnosis based on cystoscopy or imaging. Its surgical treatment depends on the operative finding. In this report, the authors report a case of gangrenous cystitis in a 73-year-old patient with a history of prostatectomy and radiotherapy whose main manifestation was urinary retention. The diagnosis was made by computed tomography scan. Surgical exploration confirmed the diagnosis, thus imposing a partial cystectomy with an omentum base plasty. Despite a good postoperative clinical and biological evolution, the patient died in the intensive care unit following respiratory distress and ventricular tachycardia. This case reminds once again the high mortality associated with this pathology.