Aims The effect of changes in serum sodium levels on the survival of patients with heart failure (HF) is unclear. We aimed to analyse the impact of serum sodium level trajectories on survival in intensive care unit (ICU) patients with HF. Methods A total of 4760 patients diagnosed with HF between 2001 and 2012 from the Medical Information Mart for Intensive Care III (MIMIC-III) database were extracted. Of these patients, 1132 patients who died within 48 h of ICU admission were excluded, and 3628 patients were included in this retrospective cohort study. Sodium levels were measured at baseline, 6, 12, 18, 24, 30, 36, 42, and 48 h. Patients were divided into hyponatremia, normal, and hypernatremia groups based on baseline sodium levels, and trajectory modelling was performed for each group separately. Group-based trajectory model (GBTM) method was utilized to identify serum sodium levels trajectories. Results The number of patients with hyponatremia (<135 mmol/L), normal sodium levels (135-145 mmol/L), and hypernatremia (>145 mmol/L) at baseline were 594 (16.37%), 2,738 (75.47%), and 296 (8.16%), respectively. A total of seven trajectory groups were identified, including hyponatremia-slow rise group [initial levels (IL), 128.48 +/- 5.42 mmol/L; end levels (EL), 131.23 +/- 3.83 mmol/L], hyponatremia-rapid rise to normal group (IL, 132.13 +/- 2.18 mmol/L; EL, 137.46 +/- 3.68 mmol/L), normal-slow decline group (IL, 137.65 +/- 2.15 mmol/L; EL, 134.50 +/- 2.54 mmol/L), normal-steady-state group (IL, 139.20 +/- 2.26 mmol/L; EL, 139.04 +/- 2.58 mmol/L), normal-slow rise group (IL, 140.94 +/- 2.37 mmol/L; EL, 143.43 +/- 2.89 mmol/L), hypernatremia-rapid decline to normal group (IL, 146.31 +/- 1.98 mmol/L; EL, 140.71 +/- 3.61 mmol/L), and hypernatremia-slow decline group (IL, 148.89 +/- 5.54 mmol/L; EL, 146.28 +/- 3.90 mmol/L). The results showed that hyponatremia-slow rise group [hazard ratio (HR) = 1.35; 95% confidence interval (CI), 1.01-1.80, P = 0.040], hyponatremiarapid rise to normal group (HR = 1.37; 95% CI, 1.11-1.71, P = 0.004), hypernatremia-rapid decline to normal group (HR = 1.46; 95% CI, 1.08-1.97, P = 0.014), and hypernatremia-slow decline group (HR = 1.49; 95% CI, 1.07-2.07, P = 0.018) trajectories were associated with an increased risk of 1-year mortality in HF patients compared with normal-steady-state group. After adjustment for all confounders, hyponatremia-rapid rise to normal group (HR = 1.26, 95% CI; 1.01-1.57, P = 0.038) and hypernatremia-rapid decline to normal group (HR = 1.36; 95% CI, 1.01-1.84, P = 0.047) trajectories were still related to an increased risk of 1-year mortality in patients with HF. Conclusions Serum sodium level trajectories were associated with mortality in patients with HF. Association between serum sodium level trajectories and prognosis in patients with HF deserve further study.