Background Heart failure (HF) and atrial fibrillation (AF) frequently coexist and may be associated with worse HF outcomes, but there is limited contemporary evidence describing their combined prevalence. We examined current trends in AF among hospitalizations for HF with preserved (HFpEF) ejection fraction or HF with reduced ejection fraction (HFrEF) in the United States, including outcomes and costs. Methods and Results Using the National Inpatient Sample, we identified 10 392 189 hospitalizations for HF between 2008 and 2017, including 4 250 698 with comorbid AF (40.9%). HF hospitalizations with AF involved patients who were older (average age, 76.9 versus 68.8 years) and more likely White individuals (77.8% versus 59.1%; P