Emphysematous cystitis (EC) is a necrotizing cystitis with gas formation. Emphysematous infections (EI) including EC easily occurs in diabetic patients and easily progress to sepsis even with immediate treatment. In addition, EI can affect anatomically adjacent organ, such as sigmoid colon, rectum, and uterus. We report an interesting case of colovesical fistula caused by emphysematous cystitis. An 84-year female was admitted with abdominal pain, diarrhea, and gross hematuria. Vital signs were initially unstable. She had a history of radiotherapy for uterine cancer. The initial CT findings showed both hydroureter and gas within the severely thickened bladder wall. She immediately received empirical antibiotics and continuous renal replacement therapy because of uncontrolled metabolic acidosis and progressive oliguria despite aggressive medical treatment. Her condition gradually improved after 2 weeks of intensive medical care, but abruptly aggravated with fecaluria and fever within a few days. The follow-up CT showed the sigmoid colovesical fistula and irregular thickness of bladder wall. We performed bilateral percutaneous nephrostomy (PCN) for urinary diversion. She improved with antibiotic therapy, PCN, and surgical treatment consisting of sigmoid colon segmental resection and loop ileostomy, but her renal function did not completely recover. In severe EI, bacterial fermentation can affect not only the urinary tract but also anatomically adjacent organs and cause serious complications such as colovesical fistula. Surgical correction is usually needed for colovesical fistula because it increases morbidity and mortality. If EI patients have complications such as colovesical fistula, temporary urinary diversion such as PCN may be helpful before surgical treatment. Thus, clinicians should monitor for the development of serous complications such as colovesical fistula if patients with EI are elderly or have a history of radiation therapy.