The spleen is an important immune organ that releases erythrocytes and monocytes and destroys aged platelets. It also reserves 20–30% of the total blood volume, and its size decreases in hypovolemic shock. However, the clinical significance of splenic size in patients with heart failure (HF) remains unclear. We retrospectively analyzed the data of 206 patients with clinically stable HF gathered between January 2001 and August 2020 and recorded in a single-center registry. All patients underwent right heart catheterization and computed tomography (CT). Splenic size was measured using CT volumetry. The primary outcomes were composite cardiac events occurring for the first time during follow-up, namely, cardiac death and hospitalization for worsening HF. The median splenic volume and splenic volume index (SVI) were 118.0 mL and 68.9 mL/m², respectively. SVI was positively correlated with cardiac output (r = 0.269, P < 0.001) and stroke volume (r = 0.228, P = 0.002), and negatively correlated with systemic vascular resistance (r =  − 0.302, P < 0.001). Seventy cardiac events occurred, and the optimal receiver operating characteristic curve SVI cutoff value for predicting cardiac events was 68.9 mL/m². The median blood adrenaline concentration was higher in the low-SVI group than the high-SVI group (0.039 ng/mL vs. 0.026 ng/mL, respectively; P = 0.004), and the low-SVI group experienced more cardiac events (log-rank test, P < 0.001). Multivariate Cox proportional hazards regression revealed that a low SVI was an independent predictor of cardiac events, even when adjusted for the validated HF risk score, blood–brain natriuretic peptide concentration, blood catecholamine concentrations, and hemodynamic parameters. Splenic size reflects hemodynamics, including systemic circulating blood volume status and sympathetic nerve activity, and is associated with HF prognosis.