A combination of giant hepatic hemangioma and diffuse hemangiomatosis is extremely rare in adults. Even when they are large, hemangiomas are soft and rarely compress adjacent structures. A 78-year-old man presented with abdominal pain and distension. Ultrasonography, computed tomography, and magnetic resonance imaging demonstrated a large expansile mass replacing the medial segment and caudate lobe with diffusely scattered nodules in the entire liver. The large hilar mass contained a central nonenhancing area and had a mass effect, leading to left portal vein occlusion. The image findings also revealed two unprecedented findings: left lateral segmental atrophy of the liver and recent portomesenteric vein thrombosis. The hepatic lesions were confirmed with hemangiomas by ultrasonography-guided biopsy. We diagnosed intrahepatic portal vein obstruction caused by a mass effect of giant hepatic hemangioma coexistent with diffuse hemangiomatosis, resulting in hepatic segmental atrophy and extrahepatic portal vein thrombosis.