Background and Purpose: Atrial fibrillation (AF) is the leading cause of ischemic stroke. Preventive antithrombotic use, especially for anticoagulation, reduces the incidence of ischemic stroke in patients with AF. Using data from the nationwide multicenter stroke registry, we investigated the trends of preceding antithrombotic medication use in patients with acute ischemic stroke (AIS) with AF and its association with initial stroke severity and in-hospital outcomes. Methods: This study included 6786 patients with AIS with known AF before stroke admission across 39 hospitals between June 2008 and December 2018. We collected the data on antithrombotic medication use (no antithrombotic/antiplatelet/anticoagulant) preceding AIS. Initial stroke severity was measured using the National Institutes of Health Stroke Scale, and in-hospital outcome was determined by modified Rankin Scale score at discharge. Results: During the study period, anticoagulant use continued to increase. However, nearly one-third of patients with AIS with known AF did not receive antithrombotics before stroke. Initial National Institutes of Health Stroke Scale scores varied according to preceding antithrombotic therapy ( P <0.001). It was higher in patients who did not receive antithrombotics than in those who received antiplatelets or anticoagulants (median National Institutes of Health Stroke Scale score: 8 versus 7 and 8 versus 6, respectively). Favorable outcome at discharge (modified Rankin Scale score, 0–2) was more prevalent in patients who received antiplatelets or anticoagulants ( P <0.001). Use of antiplatelets (odds ratio, 1.23 [95% CI, 1.09–1.38]) and anticoagulants (odds ratio, 1.31 [95% CI, 1.15–1.50]) was associated with a mild initial neurological deficit (National Institutes of Health Stroke Scale score ≤5) in patients with AIS with AF. Conclusions: Throughout the study period, the proportion of patients taking anticoagulants increased among patients with AIS with known AF. However, a large portion of AF patients still did not receive antithrombotics before AIS. Furthermore, prehospitalization use of anticoagulants was associated with a significantly higher likelihood of a mild initial neurological deficit and favorable outcome at discharge.