The clinical significance of elevated baseline serum potassium (K+) levels in hospitalised patients is rarely described. Hence, we performed a retrospective study assessing the significance of elevated K+ levels in a one-year admission cohort. Adult patients without hypokalaemia or end-stage renal disease were included. Adverse outcomes were all-cause mortality, hospital-acquired acute kidney injury, and events of arrhythmia. In total, 17,777 patients were included in the study cohort, and a significant difference (P < 0.001) was observed in mortality according to baseline serum K+ levels. The adjusted hazard ratios (HRs) and associated 95% confidence intervals (CIs) of all-cause mortality for K+ levels above the reference range of 3.6-4.0 mmol/L were as follows: 4.1-4.5 mmol/L, adjusted HR 1.075 (95% CI 0.981-1.180); 4.6-5.0 mmol/L, adjusted HR 1.261 (1.105-1.439); 5.1-5.5 mmol/L, adjusted HR 1.310 (1.009-1.700); >5.5 mmol/L, adjusted HR 2.119 (1.532-2.930). Moreover, the risks of in-hospital acute kidney injury and arrhythmia were higher in patients with serum K+ levels above 4.0 mmol/L and 5.5 mmol/L, respectively. In conclusion, increased serum K+ levels, including mild elevations may be related to worse prognosis. Close monitoring and prompt correction of underlying causes or hyperkalaemia itself is warranted for admitted patients.