Objective To explore the association between red cell volume distribution width (RDW) and the 28‑day prognosis of patients with sepsis‑associated delirium (SAD) following intensive care unit (ICU) admission. Methods According to the Medical Information Mart for Intensive Care Ⅳ (MIMIC‑Ⅳ) Database, patients who were first admitted to ICU and diagnosed with SAD from 2008 and 2019 were selected. The optimal cutoff value for RDW was determined by receiver operating characteristic curve analysis, and SAD patients were divided into two groups: a low RDW group (RDW≤16.15, n=4 014) and a high RDW group (RDW>16.15, n=1 270). Kaplan‑Meier curve was plotted to construct survival curves for patients within 28 d of ICU admission. The association between RDW and patient prognosis was assessed by multivariate Cox regression analysis. Furthermore, the nonlinear relationship between RDW levels in SAD patients and the risk of all‑cause mortality within 28 d of ICU admission was explored using restricted cubic splines. Results A total of 5 284 patients diagnosed with SAD were included in the study, with an observed all‑cause mortality rate of 14.5% within 28 d of ICU admission. The high RDW group exhibited a significantly higher mortality rate compared to the low RDW group (25.0% vs 11.2%, P<0.001). The Kaplan‑Meier curve demonstrated a substantial reduction in cumulative survival rate at 28 d following ICU admission among patients in the high RDW group, as compared to the low RDW group (P<0.001). Multivariate Cox regression analysis revealed that elevated RDW (>16.15) independently contributed to ICU mortality in patients with SAD, with a hazard ratio (HR) of 1.90 [(95% confidence interval (CI) 1.64, 2.19), P<0.001]. This association remained statistically significant even after adjusting for covariates such as age, sex, ethnicity, comorbidities, signs, scores, and laboratory measures [HR 1.54 (95% CI 1.30,2.83), P<0.001]. Furthermore, a nonlinear trend was observed, indicating an increased risk of all‑cause mortality at 28 days following ICU admission with higher RDW values. Conclusions Elevated RDW is a predictor of increased short‑term mortality following ICU admission in patients with SAD.