Radiation-induced gastroduodenitis is a well-known but rare disorder causing uncontrollable hemorrhage and has not been reported as a complication of proton beam therapy in radiation treatment. Argon plasma coagulation (APC) has been shown to be effective and safe in the management of radiation-induced hemorrhagic gastroduodenitis. We describe a case of hemorrhagic gastroduodenitis after proton beam radiation therapy for pancreatic cancer with multiple hemorrhagic risk factors, which was treated successfully with APC. A 62-year-old man was diagnosed as having early pancreatic cancer that was incidentally detected on computed tomography when screening for hepatocellular carcinoma. He opted to receive radical proton beam radiation for pancreatic cancer but not surgery because he had multiple risk factors such as liver cirrhosis due to hepatitis C virus and chronic renal failure that required hemodialysis. Three months later, however, he developed hemorrhagic gastroduodenitis induced by proton beam radiation although the cancer had been eradicated. Initially, he required frequent blood transfusions, but his disease condition improved dramatically after several endoscopic treatments using APC. The patient has been free of relapse after pancreatic cancer for >2 years.