Collagenous gastritis (CG) is a rare disorder characterized by the thick collagenous subepithelial bands associated with mucosal inflammation. There have been approximately fifty reports in the literature since it was first described in 1989. According to previous reports, CG is heterogeneous and classified into two groups-(1) cases limited to the gastric mucosa in children or young adults, and (2) CG associated with collagenous colitis in elderly adults presenting with chronic watery diarrhea. In Japan, only nine previous cases were reported, and all of them were young adults. We report a case of CG with collagenous duodenitis in a 22-year-old female. She had repeated upper gastrointestinal bleeding from a Dieulafoy lesion of the fornix, but had no symptoms of malabsorption or diarrhea. Endoscopic findings revealed striking nodularity with a smooth islet-shaped normal area in the antrum and the body. The pathological findings of nodular mucosa showed the deposition of collagen bands just under the mucoepithelial lesion. In addition, she had collagenous duodenitis in part of the bulbs, and a colonoscopy showed no abnormalities. We provide a literature review of CG and collagenous gastroduodenitis without colonic involvement.