A ruptured gastric artery aneurysm is a rare but important possible cause of massive intra-abdominal or gastrointestinal haemorrhage, and carries a high risk of mortality. Although aneurysms of the gastric arteries are uncommon, emergency radiologists and clinicians should be familiar with the clinical presentation, imaging findings and pathophysiology. We present two cases of massive intra-abdominal haemorrhage and haemodynamic shock secondary to acute rupture of previously occult gastric artery aneurysm and review the relevant anatomy, imaging findings and pathophysiology of gastric and other visceral artery aneurysms. By virtue of its location in the lesser omentum, a ruptured gastric artery aneurysm may result in a typical pattern and distribution of adjacent haematoma in the upper abdomen. Our description of imaging findings highlights a characteristic epicentre of intraperitoneal haemorrhage, and its typical mass effect displacement of surrounding viscera, to aid the emergent diagnosis of gastric artery aneurysm rupture.