A 62-year-old woman was diagnosed with peritoneal dissemination of gastric cancer and was treated with anticancer drugs. Eleven months after the start of the treatment, follow-up computed tomography newly showed thickening of the bladder wall and left hydronephrosis even though the chemotherapy reduced peritoneal dissemination. Therefore, she was referred to our hospital for further evaluation. Cystoscopy and magnetic resonance imaging showed the tumor arising from the bladder neck to trigone. A few days later, she was admitted to our hospital because of bladder tamponade. Transurethral coagulation was carried out, and we resected part of the bladder tumor for pathological examination at the same time. As the pathological features of the bladder tumor were similar to those of the primary stomach cancer and peritoneal dissemination, the diagnosis of the bladder tumor was metastatic gastric adenocarcinoma. She died three months after visiting our hospital.