An adenocarcinoma of the appendix invading the urinary bladder, which is difficult to be diagnosed before the operation, is an extremely rare disease. Only a few cases have been reported. Here we reported a case of patient diagnosed with the mucinous adenocarcinoma of the appendix invading the urinary bladder. The case reported in this study was a 54-years old man who was admitted due to a 6-month history of intermittent episodes of irritative voiding symptoms of the bladder, and weight loss. The patient did not have any gastrointestinal symptoms. The physical examination, laboratory examination, cytology of the urine, computed tomography and cystoscopy were inconclusive. The partial cystectomy, subsequent exploratory laparotomy and intraoperative frozen analysis revealed the appendiceal mucinous adenocarcinoma with a fistula to the urinary bladder. The appendectomy and the right hemicolectomy with a ileocolic anastomosis, the lymphadenectomy and the partial cystectomy limited to the anterior wall was performed. Six months after operation, the patient was in a good health with no obvious discomfort, no recurrence or distant metastases. The recommended treatment for the adenocarcinoma of the appendix invading the bladder with a fistula formation is as follows: appendectomy, right hemicolectomy with ileocolic anastomosis, lymphadenectomy, partial cystectomy and intraperitoneal hyperthermic chemoperfusion.