Objective To explore the effects of different serum potassium levels on the 28-day prognosis of elderly patients with sepsis. Methods A retrospective analysis was conducted on the clinical data and laboratory indicators within 24 hours after the diagnosis in 204 elderly patients with sepsis admitted to the Department of Critical Care Medicine, Zhongshan Hospital, Fudan University from January 2018 to January 2022. According to the potassium concentrations in the blood of the patients within 24 hours after admission to intensive care unit (ICU), the patients were divided into the hyperkalemia group (K^＋≥5.3 mmol/L), normokalemia group (K^＋ 3.5–＜5.3 mmol/L), and hypokalemia group (K^＋＜3.5 mmol/L). According to 28-day outcomes, the patients were divided into the death group and survival group. The acute physiological and chronic health evaluation (APACHE) Ⅱ score, sequential organ failure assessment (SOFA) score and laboratory indicators were analyzed. Multivariate logistic regression analysis was used to analyze the risk factors for 28-day mortality in elderly patients with sepsis. Results Compared with the normokalemia group (n＝99), patients in both the hyperkalemia (n＝61) and hypokalemia (n＝44) groups had significantly higher levels of C-reactive protein (CRP) and procalcitonin (PCT), longer duration of mechanical ventilation, longer length of hospital stay and ICU stay, and higher mortality (P ＜ 0.05). The death group (n＝49) had significantly higher APACHE Ⅱ score, SOFA score, white blood cell (WBC) counts, CRP level, and PCT level than the survival group (n＝155, P＜0.05). Multivariate logistic regression analysis showed that increased or decreased serum potassium level, higher SOFA score, and increased WBC counts were independent risk factors for 28-day mortality in elderly patients with sepsis (P＜0.05). Quadratic fitted curve showed a U-shaped association between serum potassium levels and 28-day mortality risk in elderly patients with sepsis, with the lowest mortality risk observed within the intermediate (normal) range, and both hypokalemia and hyperkalemia were associated with an increased mortality risk (P＝0.182). Conclusion Both increased and decreased serum potassium levels are independent risk factors for 28-day mortality in elderly patients with sepsis, which should be given particular attention in clinical management.