Thrombophlebitis of the portal vein (pylephlebitis) is a rare but serious condition with a high mortality rate of 11-50%. A 56-year-old male patient presented with a two-day history of postprandial, colic-like epigastric pain, nausea, fever, chills, and diarrhea. Clinical workup showed peritonism, leukocytosis, and elevated C-reactive protein (CRP). A computed tomography (CT) scan revealed a long-segment, partial thrombosis of the superior mesenteric vein as well as gas in the portal venous system. Additionally, extensive jejunal diverticulosis was present. Pylephlebitis mostly results from intestinal infections, e.g., appendicitis or diverticulitis. We assumed that the patient had suffered from a self-limiting episode of jejunal diverticulitis leading to septic thrombosis. Initially, antibiotic therapy and anticoagulation with heparin were administered. The patient deteriorated, and due to increasing abdominal defense, fever, and hypotension, a diagnostic laparoscopy was performed. Bowel ischemia could be ruled out, and after changing antibiotic therapy, the patient's condition improved. He was discharged without any further complications and without complaints on day 13. An underlying coagulopathy like myeloproliferative neoplasm or antiphospholipid syndrome could be ruled out.