Aims Hyperkalaemia and hypokalaemia are common in heart failure and associated with worse outcomes. However, the optimal potassium range is unknown. We sought to determine the optimal range of potassium in patients with heart failure and reduced ejection fraction (amp;lt; 40%) by exploring the relationship between baseline potassium level and short- and long-term outcomes using the Swedish Heart Failure Registry from 1 January 2006 to 31 December 2012. Methods and results We assessed the association between baseline potassium level and all-cause mortality at 30 days, 12 months, and maximal follow-up, in uni- and multivariable stratified and restricted cubic spline Cox regressions. Of 13 015 patients, 93.3% had potassium 3.5-5.0 mmol/L, 3.7% had potassium amp;lt;3.5 mmol/L, and 3.0% had potassium amp;gt;5.0 mmol/L. Potassium 5.0 mmol/L were more common with lower estimated glomerular filtration rate and heart failure of longer duration and greater severity. The potassium level associated with the lowest hazard risk for mortality at 30 days, 12 months, and maximal follow-up was 4.2 mmol/L, and there was a steep increase in risk with both higher and lower potassium levels. In adjusted strata analyses, lower potassium was independently associated with all-cause mortality at 12 months and maximal follow-up, while higher potassium levels only increased risk at 30 days. Conclusion In this nationwide registry, the relationship between potassium and mortality was U-shaped, with an optimal potassium value of 4.2 mmol/L. After multivariable adjustment, hypokalaemia was associated with increased long-term mortality but hyperkalaemia was associated with increased short-term mortality.