Although the fibrosis-4 index (FIB-4) is associated with right atrial pressure or prognosis in acute heart failure (AHF), the prognostic impact of its reduction during hospitalization remains uncertain. We included 877 patients (age, 74.9 ± 12.0 years; 58% male) hospitalized with AHF. The reduction in FIB-4 was defined as: (FIB-4 on admission-FIB-4 at discharge)/FIB-4 on admission × 100. Patients were divided into low (< 1.0%, n = 293), middle (1.0-27.4%, n = 292), and high (> 27.4%, n = 292) FIB-4 reduction groups. The primary outcome was a composite of all-cause death or heart failure rehospitalization within 180 days. The median FIB-4 reduction was 14.7% (interquartile range - 7.8-34.9%). The primary outcome was observed in 79 (27.0%), 63 (21.6%), and 41 (14.0%) patients in the low, middle, and high FIB-4 reduction groups, respectively (P = 0.001). Adjusted Cox proportional-hazards analysis revealed that the middle and low FIB-4 reduction groups were associated with the primary outcome, independent of the pre-existing risk model including baseline FIB-4 ([high vs. middle] hazard ratio [HR]: 1.70, 95% confidence interval [CI]: 1.10-2,63, P = 0.017; [high vs. low] HR: 2.16, 95% CI 1.41-3.32, P < 0.001). FIB-4 reduction provided additional prognostic value to the baseline model, including well-known prognostic factors ([continuous net reclassification improvement] 0.304; 95% CI 0.139-0.464; P < 0.001; [integrated discrimination improvement] 0.011; 95% CI 0.004-0.017; P = 0.001). Additionally, the combination of the reduction in FIB-4 and brain natriuretic peptide was useful for risk stratification. In conclusion, among patients hospitalized with AHF, a greater FIB-4 reduction during hospitalization was associated with better prognoses.