BACKGROUND: Hemoglobin-to-red cell distribution width ratio (HRR) has shown good prognostic value in various cancers. However, the relationship between HRR and outcomes in critically ill patients with traumatic brain injury (TBI) remains unclear. This study aimed to investigate the association between HRR and mortality among critically ill patients with TBI. METHODS: The Medical Information Mart for Intensive Care-IV (MIMIC-IV) database was utilized to conduct this retrospective cohort study. TBI patients were divided into four quartiles according to their HRR values. The primary outcome was 30-day mortality, whereas the secondary outcomes were 60-day and 120-day mortality. Univariable and multivariable Cox proportional risk models were performed to evaluate the hazard ratio (HR) and 95% confidence interval (CI) for the relationship between HRR and mortality. Receiver operating characteristic (ROC) curves were conducted to assess the prognostic value of HRR. RESULTS: For 30-day mortality, after adjustment for all potential covariates, the relationship remained significant with HRR treated as a continuous variable (HR, 95% CI: 0.87 [0.81, 0.92]; p < 0.001). In the fully adjusted model, the HR with 95% CI for the second, third, and fourth quartile groups were 0.67 (0.5, 0.9), 0.65 (0.46, 0.94), and 0.5 (0.32, 0.79), respectively, compared to the first quartile group. A similar relationship was also observed for 60-day mortality and 120-day mortality. HRR had a better predictive value than hemoglobin and red cell distribution width (RDW). CONCLUSIONS: A lower level of HRR is significantly associated with higher all-cause mortality among critically ill patients with TBI.