Background: Hyponatremia is among the most frequent electrolyte disturbances in acute decompensated heart failure (ADHF) and is a robust indicator of disease severity. Despite therapeutic advances, mortality and hospital readmissions among patients with ADHF remain high, and abnormalities in serum sodium have emerged as clinically meaningful prognostic biomarkers. Contemporary data (2022–2025) reaffirm that even mild reductions in serum sodium confer incremental risk for adverse outcomes. Objective: This study aimed to evaluate the association between admission serum sodium concentration and in-hospital clinical outcomes in patients hospitalized with Heart Failure (HF). Methods: We conducted a cross-sectional descriptive study involving 61 patients admitted with HF to two tertiary military hospitals in Yangon, Myanmar, from October 2009 to August 2011. Admission serum sodium levels, clinical characteristics, and echocardiographic parameters were recorded. Hyponatremia was defined as serum sodium <135 mmol/L. Outcomes included in-hospital mortality and HF-related complications. Results: Hyponatremia was present in 45.9% of patients. Compared with normonatremic individuals, hyponatremic patients demonstrated significantly higher rates of congestion markers, including elevated jugular venous pressure, peripheral oedema, tachycardia, hypotension, and greater left atrial diameter. In-hospital mortality was markedly higher among hyponatremic patients (14.1% vs. 3.2%). These results align with updated global evidence showing that hyponatremia independently predicts mortality, prolonged hospitalization, diuretic resistance, and increased readmission rates. Conclusion: Admission hyponatremia is a strong, independent predictor of short-term mortality in hospitalized HF patients. Recent studies emphasize that sodium abnormalities reflect neurohormonal activation and hemodynamic deterioration. Routine assessment and management of serum sodium should be integrated into contemporary HF care.