Background and aims: Pulmonary and/or peripheral venous congestion defines the clinical diagnosis of acute heart failure (AHF). However, the systolic pulmonary arterial pressure (sPAP) thresholds at which pulmonary (chest x-ray) and inferior vena cava (IVC) congestion occur are not well established. This study aimed to identify a cut-off value of sPAP that reliably indicates AHF. Methods and results: We retrospectively included 380 consecutive patients hospitalized for AHF at an Italian referral centre, after excluding those with severe tricuspid regurgitation. Receiver operating characteristic (ROC) curve analysis and Youden’s J statistic identified a threshold of sPAP ≥ 48.75 mmHg as the most accurate in predicting both pulmonary (sensitivity = 89.9%, specificity = 73%) and peripheral (sensitivity = 88.3%, specificity = 82.5%) fluid overload. The association between this sPAP threshold and both pulmonary and peripheral congestion was confirmed by chi-square testing (p < 0.001) and multivariate logistic regression (p < 0.001). After adjustment for confounders, sPAP ≥ 49 mmHg was independently associated with all-cause death or heart failure (HF) hospitalization (HR = 1.713; 95% CI 1.127–2.602; p = 0.012). Conclusions: sPAP threshold of 49 mmHg identifies AHF with clinically useful accuracy—pulmonary (chest X-ray) congestion