Obesity is well-established as a major cardiovascular (CV) risk factor. Obesity confers a greater risk for developing atrial fibrillation (AF), but the relation between obesity and established nonvalvular AF for stroke and all-cause death is still unclear. To ascertain the prevalence of overweight and obesity in patients with nonvalvular AF, their influence on adverse events, and the relation with anticoagulation control, we performed this post hoc analysis of the pooled Stroke Prevention using an Oral Thrombin Inhibitor in patients with atrial Fibrillation (SPORTIF) III and V data sets. For this study, we analyzed all patients assigned to the warfarin arm with data on body mass index (BMI). Time in therapeutic range was used as an index of the quality of anticoagulation control. The 3,630 patients eligible for this analysis were categorized as follows: (1) BMI 18.5 to 24.9 (“normal weight”) in 24.1%; (2) BMI 25.0 to 29.9 (“overweight”) in 39.8%; and BMI ≥30 (“obese”) in 36.1%. Both overweight (hazard ratio [HR] 0.70) and obese (HR 0.59) categories were inversely associated with the composite outcome of stroke/all-cause death. A similar inverse association was seen for the end point of stroke (HR 0.61 and 0.47, respectively). Good anticoagulation control also attenuated the association between BMI categories and outcomes. In patients with time in therapeutic range >70%, BMI category was not significantly associated with the composite outcome of stroke/death and stroke. Stroke and all-cause death progressively reduced in overweight and obese anticoagulated patients with AF. The inverse relation of BMI categories to the risk of stroke and all-cause death was mitigated by good anticoagulation control.