Idiopathic spontaneous intraperitoneal hemorrhage (ISIH) is a phenomenon caused by spontaneous rupture of intra-abdominal visceral vessels, and vascular malformations (VMs) leading to ISIH are rare in previously reported cases. VMs of the gastric wall, which are commonly located in the mucosa and submucosa, mostly lead to upper gastrointestinal bleeding rather than intraperitoneal hemorrhage. To our knowledge, this is the first report of ISIH caused by VMs in gastric muscularis. In the current case, a 22-year-old male patient presented with sudden abdominal pain for 4 h, accompanied by tachycardia and hypotension. CT revealed a hematoma in the omental bursa and fluids in abdominopelvic cavities. Then intraperitoneal hemorrhage was confirmed after abdominal paracentesis. Furthermore, ultrasonic gastroscopy indicated that vascular malformation in the muscularis of the stomach probably led to intraperitoneal hemorrhage. The patient recovered after conservative treatment based on fluid resuscitation and remained stable for 12 months of follow-up. This case suggests that VMs located in the gastrointestinal tract may lead to ISIH and ultrasonic gastroscopy is helpful in the diagnosis of VMs in the gastrointestinal tract.