Background. There is no evidence suggesting that red blood cell distribution width‐to‐albumin ratio (RA) predicts outcomes in severely ill older individuals with acute kidney injury (AKI). We hypothesized that RA is associated with all‐cause mortality in critically ill older patients with AKI. Methods. We recorded demographics, laboratory tests, comorbidities, vital signs, and other clinical information from the MIMIC‐III V1.4 dataset. The primary endpoint was 90‐day all‐cause mortality, and the secondary endpoints were 30‐day mortality, one‐year mortality, renal replacement treatment (RRT), duration of stay in the intensive care unit (ICU), sepsis, and septic shock. We generated Cox proportional hazards and logistic regression models to determine RA’s prognostic values and subgroup analyses to determine the subgroups’ mortality. We conducted a Pearson correlation analysis on RA and C‐reactive protein (CRP) in the cohort of patients from the Second Affiliated Hospital of Wenzhou Medical University. Results. A total of 6,361 patients were extracted from MIMIC‐III based on the inclusion and exclusion criteria. RA levels directly and linearly correlated with 90‐day all‐cause mortality. After controlling for ethnicity, gender, age, and other confounding variables in multivariate analysis, higher RA was significantly associated with an increased risk of 30‐day, 90‐day, and one‐year all‐cause mortality as opposed to the reduced levels of RA (tertile 3 vs. tertile 1: hazard ratios (HRs), 95% confidence intervals (CIs): 1.70, 1.43–2.01; 1.90, 1.64–2.19; and 1.95, 1.72–2.20, respectively). These results suggested that elevated levels of RA were linked to an elevated risk of 30‐day, 90‐day, and one‐year all‐cause death. There was a similar trend between RA and the use of RRT, length of stay in ICUs, sepsis, and septic shock. The subgroup analysis did not reveal any considerable interplay among strata. When areas under the curve were compared, RA was a weaker predictor than the SAPS II score but a stronger predictor than red blood cell distribution width (RDW) or albumin alone (P < 0.001); RA combined with SAPS II has better predictive power than SAPS II alone (P < 0.001). The Second Affiliated Hospital of Wenzhou Medical University cohort showed that CRP positively correlated with RA, with a coefficient of 0.2607 (P < 0.001). Conclusions. RA was an independent prognostic predictor in critically ill older patients with AKI, and greater RA was linked to a higher probability of death. The risk of AKI is complicated when RRT occurs; sepsis and septic shock increase with RA levels.