Catastrophic antiphospholipid syndrome is a highly fatal condition characterized by widespread thromboembolism subsequent to a triggering factor (e.g., infection, trauma, and neoplasia) in antiphospholipid antibody-positive patients. This paper reports a case of a 29-year-old male without the underlying disease who developed extensive mesenteric thromboembolism and jejunal necrosis during the treatment for acute enteritis. The patient’s condition was improved with low-molecular-weight heparin and an intravenous Ig treatment with emergency surgery. The serum antiphospholipid (anticardiolipin IgM) and lupus anticoagulant antibody tests showed positive results. Acute infectious enterocolitis is generally considered a mild disease. On the other hand, aggressive evaluation and treatment should be considered if the clinical conditions do not improve and deteriorate rapidly despite appropriate antibiotic treatment because of the possibility of acute immunological complications, such as catastrophic antiphospholipid syndrome. (Korean J Gastroenterol 2021;77:294-299)