Background and Objectives: Few data have examined cardiac function correlates of hyponatremia in heart failure (HF) patients. We examined the association between hyponatremia and echocardiographic parameters with outcomes among HF patients in the cardiac intensive care unit (CICU). Methods: Retrospective analysis of 3,372 Mayo Clinic CICU patients with HF from 2007 to 2018 grouped according to admission serum sodium: severe hyponatremia (sodium <130 mEq/L, 6%), mild hyponatremia (sodium 130–134 mEq/L, 16%), normal (sodium 135–144 mEq/L, 78%).Echocardiographic findings and mortality were compared across groups. Results: The median age was 71.8 years old, and 39% were females. Patients with hyponatremia had worse echocardiographic parameters reflecting left ventricular (LV) systolic function and forward flow, particularly stroke volume index and LV systolic work index, with higher right atrial pressure and worse right ventricular-pulmonary artery coupling. In-hospital mortality occurred in 12.4% and increased with more severe hyponatremia: ≥135 mEq/L, 10.8%; 130–134 mEq/L, 16.7%; <130 mEq/L, 21.6% (p<0.001). One-year mortality occurred in 32.1% and was higher in patients with hyponatremia: ≥135 mEq/L, 29.6%; 130–134 mEq/L, 40.7%; <130 mEq/L, 40.7% (p<0.001). Patients with hyponatremia had significantly higher in-hospital (adjusted odds ratio, 1.80; 95% confidence interval [CI], 1.44–2.25; p<0.001) and one-year (adjusted hazard ratio, 1.51; 95% CI, 1.32–1.72; p<0.001) mortality, both overall and when stratified by echocardiographic measures. Additive increases in mortality were seen in patients with hyponatremia and poor echocardiographic hemodynamics. Conclusions CICU patients with HF and hyponatremia have worse hemodynamics reflected by echocardiographic parameters of LV systolic and diastolic function and forward flow, plus worse right ventricular function, with resultant worse clinical outcomes.