Abnormal serum potassium levels (K+) in patients with heart failure (HF) relate to worse prognosis. We evaluated whether admission K+ levels predict 1-year outcomes in elderly patients admitted for acute HF. We evaluated 2865 patients aged >74 years from the RICA Spanish Heart Failure Registry, classified according to admission serum K+ levels: hyperkalemia (>5.5 mmol/L), normokalemia (3.5–5.5 mmol/L) and hypokalemia (<3.5 mmol/L). We explored whether K+ levels were significantly associated with one-year all-cause mortality or hospital readmission and their combination. Mean admission K+ value was 4.3 ± 0.6 mmol/L; 97 patients (3.38%) presented with hyperkalemia and 174 (6.06%) with hypokalemia. Overall, 43% of the patients died or were readmitted for HF during the follow-up period; the risk was higher for those with hyperkalemia (59% vs 41% in hypokalemic patients). The HR for one-year mortality was 1.43 (p = .073) and 1.67 for readmissions (p = .007) when K+ was >5.5 mmol/L and 1.08 (p = .618) and 0.90 (p = .533) respectively for K+ < 3.5 mmol/L. The HR for the combined outcome was 1.59 (1.19–2.13); p = .002 in hyperkalemic patients and 0.96 (0.75–1.23); p = .751in hypokalemic patients. Multivariate analysis showed a significant association of admission K+ values >5.5 mmol/L with the combined outcome of mortality and readmission (HR 1.15 95% CI 1.04–1.27, p = .008). In patients hospitalized for decompensated HF, admission hyperkalemia predicts a higher mid-term risk for HF readmission and mortality, probably related to the significant higher risk of readmission. •Abnormal serum potassium (K+) values may be associated with poor health results in heart failure (HF) patients.•At admission, 3.38% of patients presented with hyperkalemia and 6.06% with hypokalemia.•Hyperkalemia at admission predicts a higher one-year-term risk for HF readmission and mortality.•Probably this association is related to the significant higher risk of readmission.