Background and Purpose- Aortic arch atherosclerosis (AAA) is a possible source of embolism in patients with embolic stroke of undetermined source. Previous studies reported high rates of embolic events in patients with AAA, especially those with high-risk AAA. This exploratory analysis of NAVIGATE ESUS (New Approach Rivaroxaban Inhibition of Factor Xa in a Global Trial Versus ASA to Prevent Embolism in Embolic Stroke of Undetermined Source) focused on patients with AAA and assessed their characteristics, stroke recurrence rates, and response to treatment. Methods- The detection of AAA and the assessment of its features were based on transesophageal echocardiography that was done in 19% of participants. AAA plaques were considered to have complex features when reported as complex or ulcerated or were ≥4 mm in thickness or had a mobile thrombus present. Results- Among 1382 participants who had transesophageal echocardiography, 397 (29%) had AAA and 112 (8%) had complex AAA. Mean (SD) age (63 [10] versus 67 [9] versus 69 [9]; P<0.001), prevalence of diabetes mellitus (19% versus 26%, versus 32%; P=0.002), and aortic valvulopathy (10 versus 20 versus 20; P<0.001) increased across no versus noncomplex versus complex AAA, respectively. In multivariable analyses, increasing age, diabetes mellitus, aortic valvulopathy, statin use before randomization, chronic infarcts on imaging, and region were independently associated with any AAA versus no AAA and also with complex AAA versus no AAA. Multiterritorial qualifying infarcts rather than single-territory infarcts were observed in 21% with complex AAA versus 17% noncomplex versus 13% no AAA (P=0.07). Annualized rates of ischemic stroke recurrence were 7.2% versus 4.2% versus 5.6% for complex versus noncomplex versus no AAA, respectively. While prevalence of complex AAA increased with increasing risk score, after adjusting for risk score, we did not observe increased risk of recurrent stroke for patients with complex AAA (hazard ratio, 1.1; 95% CI, 0.53-2.4), although the number of outcomes was limited. In patients with complex AAA, 4 strokes occurred among rivaroxaban-assigned patients and 4 strokes among aspirin-assigned patients. Conclusions- Complex AAA is prevalent in embolic stroke of undetermined source patients and is associated with atherosclerotic burden. Whether complex AAA independently increases recurrent stroke risk and whether a non-vitamin-K oral anticoagulant as compared with aspirin may be effective for reducing recurrent stroke requires additional study. Clinical Trial Registration- URL: https://www.clinicaltrials.gov. Unique identifier: NCT02313909.