Autoamputation of a gastric polyp is a relatively rare phenomenon and its precise mechanism is unclear. To learn more about the mechanism(s) involved, it is important to observe a polyp just before and just after its disappearance. We report a case of a gastric polyp that was observed endoscopically just before and then just after autoamputation. A 61-year-old woman with a thumb-sized, pedunculated hyperplastic polyp in the gastric antrum visited our institution for investigation of hematemesis. She was being treated with oral hypoglycaemic drugs for diabetes mellitus but was not taking any other medicine around that time. Emergency gastroscopy revealed a bleeding point near the polyp; gastroscopy the next day revealed that the polyp had disappeared. It was concluded that autoamputation of a gastric polyp may follow gastric injury induced by diabetes mellitus or oral antidiabetic drugs.