Among the fatal vascular complications associated with autosomal dominant polycystic disease (ADPKD), ruptured intracerebral aneurysm and ruptured abdominal aortic aneurysm are widely known. However, there are few reports on the dissecting thoracic aortic aneurysm as a fatal complication of ADPKD. We report a case of a 58-year-old man with a history of ADPKD who presented to the emergency department with out-of-hospital cardiac arrest. Immediate cardiopulmonary resuscitation restored a spontaneous circulation successfully and subsequent image study revealed a type I dissecting thoracic aortic aneurysm. Emergency aortic grafting was performed--but he died from postoperative haemorrhage. The surgical specimen of the aorta showed cystic medial necrosis. This rare case emphasizes the need to consider such a diagnosis in a patient with ADPKD who presents to the emergency department with sudden cardiac arrest. In addition, the histological finding indicates the aetiological role of a collagen defect in addition to chronic hypertension in the pathogenesis of aortic dissection in ADPKD patients.