Hemorrhagic radiation cystitis is an example of a typical radiotherapy-induced adverse event. However, the optimal treatment for hemorrhagic radiation cystitis is not known. There are limited data regarding the use of argon plasma coagulation for hemorrhagic radiation cystitis. Here, we present the use of argon plasma coagulation using a gastrointestinal endoscope to treat hemorrhagic radiation cystitis. The patient was a 75-year-old male patient with hemorrhagic radiation cystitis due to external beam irradiation for prostate adenocarcinoma. Six years after radiotherapy, the patient presented with macroscopic hematuria over the preceding 4 months, and laboratory investigations revealed a low hemoglobin level. The hematuria was not controlled with 2 days of bladder irrigation using normal saline. Thus, argon plasma coagulation using an upper gastrointestinal endoscope was considered for treatment of the hemorrhagic radiation cystitis. The cystoscopic examination revealed diffuse radiation cystitis with oozing telangiectasia and coagula. All of the bleeding sites and telangiectasia were coagulated using argon plasma coagulation. Following treatment, the patient's clinical symptoms improved and did not recur. The hemoglobin level also recovered. No complications associated with the treatment were observed during the 6-month follow-up period. Thus, argon plasma coagulation using a gastrointestinal endoscope is a safe and effective treatment for hemorrhagic radiation cystitis.