A gastric ulcer was detected in a 54-year-old man who underwent upper gastrointestinal endoscopy for hematemesis. An abdominal contrasted computed tomography scan detected a splenic artery aneurysm adjacent to the gastric wall. Endoscopic hemostasis was thought to be risky owing to possible rupture of the aneurysm. Rupture of a splenic artery pseudoaneurysm due to segmental arterial mediolysis (SAM) was diagnosed by abdominal angiography, and subsequently transcatheter arterial embolization was performed. In cases of upper gastrointestinal hemorrhage, hemostasis is often performed during the emergency endoscopic examination. However, in cases of massive gastrointestinal bleeding, the possibility of a splenic artery aneurysm, in association with SAM, should be recognized. The risk of rupturing the aneurysm should be considered in selecting the most suitable treatment.