The authors report the case of a patient with isolated protein C deficiency detected later in life, presenting with a mobile aortic thrombus and splenic infarction. Only one such case has been previously described. This case emphasizes the importance of including the aorta in the search for a cause of systemic embolization and highlights the diagnostic options and management dilemmas. Although anticoagulation with subsequent reassessment of thrombus size can be considered for layered thrombi, mobile thrombi warrant early surgical intervention to minimize the risk for systemic embolization. This patient was treated surgically with encouraging results.