Background: Heart failure is associated with substantial morbidity and mortality. Patients with heart failure often exhibit hyponatremia and may reflect hemodynamic and neurohormonal disturbances. Aim of study: To evaluate the role of serum sodium level as a predictor of mortality among patients with HF in Iraq. Methods: A prospective cohort study was conducted among 100 adult patients with heart failure admitted to the Internal Medicine Department of Baghdad Teaching Hospital between July 2025 and July 2026. It included 100 adult patients who presented to the adult patients with HF hospitalized to the hospital with left ventricular ejection fraction < 49% at admission and followed up to evaluate in-hospital mortality. Demographic and clinical characteristics, laboratory parameters, ejection fraction, duration of hospitalization, and patient outcome were recorded. Results: In this study, hyponatremia was present in 58% of patients. Overall, 32% of patients died. Mortality was significantly higher among patients with hyponatremia than among those with normal serum sodium levels (43.1% vs. 16.7%, P=0.005). Patients with hyponatremia also had a significantly longer duration of hospitalization, lower systolic and diastolic blood pressures, and higher mean heart rate than patients with normal sodium levels (P < 0.001).  Conclusions: Serum sodium is an inexpensive, readily available clinical marker for risk stratification and prognostic assessment in patients with heart failure as hyponatremia is common among hospitalized patients with heart failure and is significantly associated with increased mortality.