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Background: The kidneys play a central role in serum potassium (K+) homeostasis, and their dysfunction leads to electrolyte disorders. We aimed to examine the relationship between different levels of K+ and mortality among very elderly patients with acute kidney injury (AKI). Methods: We retrospectively enrolled very elderly patients (≥75 years) with AKI from the hospital information system of the Chinese PLA General Hospital from January 1, 2007 to December 31, 2018. All-cause mortality was examined according to six predefined K+ levels: <3.50 mmol/L, 3.50–3.79 mmol/L, 3.80–4.09 mmol/L, 4.10–4.79 mmol/L, 4.80–5.49 mmol/L, and ≥5.50 mmol/L. We estimated the risk of all-cause mortality using the multivariable adjusted Cox proportional hazard model with the normal K+ level at 3.50–3.79 mmol/L as a reference. Results: In total, 747 patients were deemed suitable for the final evaluation. The median age of the 747 participants was 88 (84–91) years. After 90 days, the mortality rates in the six strata were 28.3%, 21.9%, 30.1%, 35.4%, 45.2%, and 58.3%, respectively. In the multivariable adjusted analysis, patients with K+ levels of 4.10–4.79 mmol/L (hazard ratio [HR]: 1.638; 95% confidence interval [CI]: 1.016–2.642), 4.80–5.49 mmol/L (HR: 2.585; 95% CI: 1.524–4.384), and ≥5.50 mmol/L (HR: 2.587; 95% CI: 1.495–4.479) had an increased risk of all-cause mortality. After 1 year, the mortality rates in the six strata were 44.8%, 41.1%, 45.1%, 51.8%, 63.1%, and 76.3%, respectively. In the multivariable adjusted analysis, patients with K+ levels of 4.10–4.79 mmol/L (HR: 1.452; 95% CI: 1.014–2.079), 4.80–5.49 mmol/L (HR: 2.151; 95% CI: 1.427–3.241), and ≥5.50 mmol/L (HR: 2.341; 95% CI: 1.514–3.620) had an increased risk of all-cause mortality. Conclusion: Increased serum K+ levels, including levels of 4.10–5.49 mmol/L and ≥5.50 mmol/L, were associated with a significantly increased short- and long-term risk of death. Serum K+ has the potential to be a marker of disease severity among very elderly patients with AKI.