Antiphospholipid syndrome (APS) is an autoantibody-mediated acquired thrombophilia characterized by venous and/or arterial thromboses, pregnancy morbidity (predominantly repeated fetal losses), and the presence of phospholipid antibodies. The estimated annual incidence of APS is 5 new cases per 100,000 people. The most common thrombotic events in patients with APS in order of frequency are stroke, transient ischemic attack, deep vein thrombosis, and pulmonary embolism. Patients with APS may develop an intracardiac thrombus, which is a life-threatening complication with a high risk of increased morbidity and mortality; however, it is treatable by surgical removal, extensive anticoagulant administration, and prevention of other complications. Catastrophic APS, which is a rare and severe condition diagnosed based on rapidly progressive thromboembolic events involving three or more organs, systems, or tissues, occurs in less than 1% of all patients with APS. We herein report an autopsy case of catastrophic APS in a 12-year-old Thai boy with multiple thromboembolic events including intracardiac thrombus formation with a positive lupus anticoagulant test result. To the best of our knowledge, this is the youngest reported patient with APS to date.