Background: The pan-immune–inflammation (PIV) score is a hematological index derived from neutrophil, platelet, monocyte and lymphocyte counts. It has been demonstrated that it has high prognostic value in oncological patients. The aim of this study was to evaluate the association between PIV and 28-day mortality in elderly (≥65 years) critically ill patients admitted to the intensive care unit (ICU) with a diagnosis of sepsis. Methods: This single-centre retrospective study included 96 patients aged ≥65 years who were admitted to the ICU with a diagnosis of sepsis according to the Sepsis-3 criteria between 15 July 2024 and 15 July 2025. Patients were divided into low- and high-PIV groups based on the median PIV. Cox proportional hazards regression analysis and Kaplan–Meier survival analysis were performed. Results: The overall 28-day mortality rate was found to be 55.2% (n = 53). The median PIV was 866.58 (IQR: 497.34–1978.43). The PIV was shown not to be a significant predictor of 28-day mortality (AUC: 0.550; p = 0.400). No difference in survival was observed between the low- and high-PIV groups in the Kaplan–Meier analysis (log-rank p = 0.662). In multivariate Cox regression, high creatinine (HR: 2.683; p < 0.001), high calcium (HR: 2.312; p = 0.004), a low partial thromboplastin time (HR: 0.396; p = 0.005) and a requirement for vasopressors (HR: 2.225; p = 0.025) were identified as independent predictors of mortality. In the Kaplan–Meier analysis for 28-day survival, chronic obstructive pulmonary disease (p = 0.023) and chronic renal disease (p = 0.034) were found to be significantly associated with poorer survival. Conclusions: The PIV is unable to predict 28-day mortality in elderly critically ill patients diagnosed with sepsis. This finding suggests that immunosenescence and inflammaging reduce the predictive power of composite hematological indices. Markers of organ dysfunction, coagulopathy and hemodynamic instability remain more reliable prognostic indicators in geriatric patients with sepsis.