BACKGROUND The association of hyponatremia with cognitive impairment and mobility in heart failure (HF) patients is unknown. The purpose of this study was to determine if hyponatremia is associated with cognitive and mobility impairment as measured by simple, validated, and time-sensitive tests. MATERIAL AND METHODS This was a prospective study in patients with reduced and preserved ejection fraction (HFrEF, HFpEF) seen in outpatient HF clinics. Hyponatremia was defined as sodium level ≤136 mEq/L. Cognitive function was measured using the Montreal Cognitive Assessment (MoCA) tool, and mobility was measured with the Timed Up and Go test (TUG-t). RESULTS A total of 121 patients were evaluated; 30% were hyponatremic (134±1.9 mEq/l, range 128-136 mEq/l). Overall, 92% of hyponatremic patients had cognitive impairment (MoCA <26) compared to 76% of the non-hyponatremic patients [relative risk 1.2 (confidence interval: 1.02-1.4, p=0.02)]. In regard to mobility, 72% of hyponatremic patients and 62% of non-hyponatremic patients (p=0.4) had TUG-t times that were considered to be worse than average. A total of 84% (N=76) of HFrEF and 71% (N=22) of HFpEF patients had cognitive impairment (p=0.86). HFrEF patients had significantly lower overall MoCA scores (21.2±3.7 vs. 23.3±3.6, p=0.006) and similar TUG-t times compared to HFpEF patients. CONCLUSIONS Most heart failure patients (HFrEF and HFpEF) seen in an ambulatory setting had impairment of cognitive function and mobility, with a higher prevalence among those with hyponatremia. Screening can be done using tests that can be administered in a clinical setting.