BACKGROUND Atrial fibrillation (AF) and heart failure (HF) each contributes to global disease burden and can coexist. The interplay of prior HF, prior AF, and presenting rhythm have not previously been jointly considered in prognostic implication. OBJECTIVES The authors sought to assess 1-year all-cause mortality according to permutations of prior HF, prior AF, and AF as presenting rhythm, in a global cohort of patients hospitalized for HF. METHODS REPORT-HF enrolled patients during hospitalization for acute HF from 44 countries over 6 continents. Cox proportional hazard models were used to compute HRs for the primary outcome of 1-year all-cause mortality. RESULTS Of 13,401 participants (median age 67 years, 61% men), 58% had prior HF. AF prevalence (prior or newly detected) at HF admission was 39%, varying by left ventricular ejection fraction and race subgroups. Compared with patients with no prior HF, no prior AF, and presenting in sinus rhythm, 1-year all-cause mortality was elevated in patients with prior HF, prior AF, and presenting in AF (adjusted HR: 1.54 [95% CI: 1.34-1.78]; P < 0.001) and in patients with prior HF, no prior AF, and presenting in AF (adjusted HR: 1.51 [95% CI: 1.20-1.90]; P < 0.001), but not in patients with no prior HF and with prior AF or presenting in AF. These results were conserved across left ventricular ejection fraction and race subgroups. CONCLUSIONS In a global cohort of patients hospitalized for HF, permutations of prior HF, prior AF, and AF as presenting rhythm differentiate outcome. History of prior HF influences the prognostic implications of AF in patients hospitalized for HF. (Global Noninterventional Heart Failure Disease Registry [REPORT-HF]; NCT02595814) (JACC Heart Fail. 2025;13:453-464) (c) 2025 by the American College of Cardiology Foundation.