A retrospective cohort study investigated the relationship between red blood cell to albumin distribution width ratio (RAR) and prognosis in patients with sepsis-associated acute kidney injury (SA-AKI) in the intensive care unit of Baise People's Hospital from May 2022 to May 2025. The primary endpoint was 28-day all-cause mortality. A linear association between RAR and mortality was observed (non-linearity p = 0.162). After multivariable adjustment, the highest quartile of RAR (Q4: >8.31) was associated with a hazard ratio of 7.52 (95% CI: 2.24–25.29) versus Q1 (<5.07). Kaplan-Meier analysis showed a significantly higher mortality in the high RAR groups (p < 0.001). The area under the ROC curve for RAR in predicting 28-day mortality was 0.694 (95% CI: 0.612–0.776). Combining RAR with lactate to albumin ratio (LAR) increased the predictive accuracy to AUC 0.777 (95% CI: 0.703–0.851; p = 0.043 vs. RAR alone).