Background and Purpose— Women with atrial fibrillation (AF) have a higher risk of stroke than men who have AF. Atrial fibrosis is a marker of atrial disease that precedes the appearance of AF increasing the risk of ischemic stroke. We aimed to determine whether female sex is independently associated with left atrial fibrosis in stroke patients. Methods— We prospectively included a consecutive sample of ischemic stroke patients aged over 50 years of age. Late gadolinium enhancement cardiac magnetic resonance imaging was performed to quantify the severity of left atrial fibrosis and the wall pattern of its distribution. A multivariable linear regression analysis was performed to determine whether female sex was independently associated with left atrial fibrosis after adjusting for potential confounders namely AF and age. Results— One hundred twenty-four patients were deemed eligible; 117 patients were included (7 were excluded because of cardiomyopathy identified by cardiac magnetic resonance imaging). All had usable cardiac magnetic resonance imaging data. Fifty-three patients (45.3%) were women. Women were older and were less frequently treated with angiotensin-converting-enzyme inhibitors. Ninety-one patients had any degree of atrial fibrosis. Women had a higher percentage of atrial fibrosis than men—median (interquartile range)—18% (17) versus 10% (20). In a multivariable linear regression model adjusted for demographics, medications, AF, comorbidities, and cardiac parameters, female sex was found to be independently associated with left atrial fibrosis. Women were found to have more 4.70% of left atrial fibrosis than men (95% CI, 0.70–8.71%; P =0.02) after controlling for confounders. Conclusions— Female sex was found to be independently associated with left atrial fibrosis after controlling for confounders such as AF and age. Further studies are needed to understand if this contributes to the increased stroke risk related to AF in women compared with men.