A 68-year-old man presented with abdominal pain, vomiting, and diarrhea. He was admitted with a provisional diagnosis of acute gastroenteritis based on physical examination, blood tests, ultrasonography, and plain computed tomography. Despite treatment with fasting and fluid replacement, cardiopulmonary arrest occurred the following morning. All attempts at resuscitation failed and the patient died. Pathological autopsy revealed the presence of clots within the superior mesenteric vein, which was diagnosed as superior mesenteric vein thrombosis. This rare disease often manifests with only nonspecific symptoms and is typically difficult to diagnose. Because delayed diagnosis may lead to unfavorable outcomes, it is important to include this disease in the differential diagnosis of abdominal pain.