BACKGROUND: Acute decompensated heart failure (ADHF) is one of the leading causes of mortality, highlighting the importance of early identification of high-risk patients. The fibrosis-5 (FIB-5) index, traditionally used to evaluate hepatic fibrosis, may hold prognostic value in ADHF patients by reflecting systemic congestion, inflammation, and organ dysfunction. The hypothesis of this study is that the FIB-5 index is an independent predictor of 1-month mortality in patients with ADHF. METHODS: This retrospective study included 155 patients diagnosed with ADHF between 2020 and 2024. Patients were divided into two groups based on their left ventricular ejection fraction (LVEF ≤ 40% or LVEF > 50%). Survival was monitored for one month, and clinical, biochemical, and echocardiographic parameters were compared between survivors and death. Logistic regression and receiver operating characteristic curve analyses were performed to assess the prognostic value of the FIB-5 index. RESULTS: During the 1-month follow-up, 66 patients (42.6%) died. The mean FIB-5 index was significantly lower in non-survivors (-10.46 ± 6.93) compared to survivors (-8.10 ± 6.67) (P = 0.03). Multivariate regression analysis identified the FIB-5 index as an independent predictor of 1-month mortality (OR = 1.089, 95% CI: 1.022-1.160, P = 0.009). The receiver operating characteristic curve analysis demonstrated an area under the curve of 0.609 (95% CI: 0.51-0.699) with sensitivity of 59.6% and specificity of 63.4%. Kaplan-Meier survival analysis revealed significantly higher mortality rates among patients with lower FIB-5 values (log-rank: 7.887, P = 0.005). CONCLUSIONS The FIB-5 index is an independent predictor of 1-month mortality in ADHF patients. Its low cost, non-invasive nature, and ability to reflect systemic inflammation and congestion make it a promising tool for risk stratification. Prospective studies are needed to validate its utility in clinical practice and evaluate its role in guiding therapeutic decisions.