Sodium picosulfate/magnesium citrate (SPMC) is a widely used oral bowel cleansing agent considered to be relatively safe. However, partially dissolved or undissolved SPMC powder may cause severe injuries of the esophagus and stomach. We report a very rare case of acute gastric injury without esophageal damage caused by the ingestion of undissolved SPMC powder. A 69-year-old man experienced epigastric pain after swallowing SPMC powder without dissolving it in water in preparation for a screening colonoscopy. He realized his mistake immediately and subsequently drank 2 L of water. The esophagogastroduodenoscopy conducted after 12 hours indicated an acute gastric ulceration without injury of the esophagus or duodenum. The endoscopy conducted after 6 weeks of oral proton pump inhibitor treatment showed healing of the gastric injury. This suggested that drinking large amounts of water after ingesting partially dissolved or undissolved SPMC powder can prevent serious esophageal injury, but offers no preventive benefit for acute gastric injury.