This study aimed to determine the optimal cut-off value of the early drop in systolic blood pressure (SBP) for worsening renal function (WRF) in hospitalized patients with heart failure (HF) and analyze predictors of WRF and the early drop in SBP at that threshold. We retrospectively enrolled 396 patients with acute decompensated HF. The early drop in SBP was defined as the difference between baseline and SBP measured 24 h after hospitalization. We performed receiver operating characteristic (ROC) analysis to determine the optimal cut-off value of the early drop in SBP for WRF and evaluated the effect of the early drop in SBP on in-hospital mortality by multivariate logistic regression analyses. The mean age of the patients was 73.4 ± 14.7 years, and 61.2% were men. A 14.0% drop in SBP was identified as the optimal cut-off value for WRF from the ROC curve analysis. An early drop in SBP ≥ 14.0% was associated with WRF in multivariate logistic regression analysis (odds ratio 7.84; 95% confidence interval 4.06–15.14; P < 0.0001). The dose of intravenous furosemide within 24 h of admission was one of the predictors of the early drop in SBP ≥ 14.0%, while no early drop in SBP was a predictor of in-hospital mortality in multivariate logistic regression models. In conclusion, the optimal cut-off value for WRF in patients with HF was a 14.0% drop in SBP within 24 h of admission. The early drop in SBP ≥ 14.0% was one of the predictors of WRF in patients with HF. However, no early drop in SBP was associated with in-hospital mortality. This study was registered with the University Hospital Medical Information Network in Japan (UMIN000035989).