Abstract Background and Aims Hyponatremia is often observed in patients with heart failure. The proposed pathophysiology of hyponatremia in heart failure is due to increase of arginine vasopressin activity that elevates renal water reabsorption, thereby diluting sodium content in plasma. Hyponatremia has been identified as a factor that worsens heart failure patients' prognosis, including cardiovascular morbidity and mortality. Thus, this study was conducted to investigate the impact of hyponatremia in heart failure patients on rehospitalisation within thirty days. Methods and Results This is a single-centre prospective cohort study. A total of 205 acute decompensated heart failure patients admitted to National Cardiovascular Center Harapan Kita from July 2018 to March 2019 were included into the study. The study population was then divided into two groups based on predischarge sodium level. In this study, hyponatremia was defined as serum sodium level less than 135 mEq/l. Among the study population, 58 patients (28.3%) had hyponatremia at discharge. The data was analyzed using chi-square test. There were 19 patients who were readmitted within 30 days and 52.9% of them had hyponatremia at discharge. The effect of hyponatremic status at discharge to 30-day readmission rate was statistically significant (p = 0.03, OR = 3.19 95%CI 1.22-8.33). Conclusion Hyponatremia in heart failure patients at discharge is associated with increased risk of 30-day readmission.