Background: The optimal antithrombotic strategy for patients with atrial fibrillation (AF) who develop acute coronary syndrome (ACS) and/or the need for percutaneous coronary intervention (PCI) is uncertain. The risk of bleeding is a major concern when oral anticoagulation is required to prevent stroke, and concomitant therapy with antiplatelet agents is required to minimize recurrent ischemic events. Design: AUGUSTUS is an international, multicenter randomized trial with a 2 × 2 factorial design to compare apixaban with vitamin K antagonists and aspirin with placebo in patients with AF who develop ACS and/or undergo PCI and are receiving a P2Y12 inhibitor. Patients will be evaluated for eligibility during their ACS and/or PCI hospitalization. The primary outcome is the composite of major and clinically relevant nonmajor bleeding defined by the International Society on Thrombosis and Haemostasis. A key secondary outcome is the composite of all-cause death and all-cause hospitalization. Other secondary objectives are to evaluate ischemic outcomes including the composite of death, myocardial infarction, stroke, stent thrombosis, urgent revascularization, and all-cause hospitalization and each individual component. The aim is to enroll approximately 4,600 patients from around 500 sites in 33 countries. AUGUSTUS will provide insight into the optimal oral antithrombotic therapy strategy for patients with AF and concomitant coronary artery disease. The unique 2 × 2 factorial design will delineate the bleeding effects of various anticoagulant and antiplatelet therapies and generate evidence to guide the selection of the optimal antithrombotic regimen for this challenging group of patients. It is the largest and only prospective randomized trial to investigate in a blinded fashion the risk and benefits of aspirin on top of a non–vitamin K antagonist oral anticoagulant and P2Y12 receptor inhibition.