Introduction: Hypotension is known to be associated with increased mortality in severe traumatic brain injury (TBI) patients. Systolic blood pressure (SBP) of <90 mmHg is the threshold for hypotension in consensus TBI treatment guidelines; however, evidence suggests hypotension should be defined at higher levels for these patients. Our objective was to determine the influence of hypotension on mortality in TBI patients requiring ICU admission using different thresholds of SBP on arrival at the emergency department (ED). Methods: Retrospective cohort study of patients with severe TBI (Abbreviated Injury Scale Head score ≥3) admitted to ICU at the QEII Health Sciences Centre (Halifax, Canada) between 2002 and 2013. Patients were grouped by SBP on ED arrival ( <90 mmHg, <100 mmHg, <110 mmHg). We performed multiple logistic regression analysis with mortality as the dependent variable. Models were adjusted for confounders including age, gender, Injury Severity Score (ISS), injury mechanism, and trauma team activation (TTA). Results: A total of 1233 patients sustained a severe TBI and were admitted to the ICU during the study period. The mean age was 43.4 ± 23.9 years and most patients were male (919/1233; 74.5%). The most common mechanism of injury was motor vehicle collision (491/1233; 41.2%) followed by falls (427/1233; 35.8%). Mean length of stay in the ICU was 6.1 ± 6.4 days, and the overall mortality rate was 22.7%. SBP on arrival was available for 1182 patients. The <90 mmHg group had 4.6% (54/1182) of these patients; mean ISS was 20.6 ± 7.8 and mortality was 40.7% (22/54). The <100 mmHg had 9.3% (110/1182) of patients; mean ISS was 19.3 ± 7.9 and mortality was 34.5% (38/110). The <110 mmHg group had 16.8% (198/1182) of patients; mean ISS was 17.9 ± 8.0 and mortality was 28.8% (57/198). After adjusting for confounders, the association between hypotension and mortality was 2.22 (95% CI 1.19-4.16) using a <90 mmHg cutoff, 1.79 (95% CI 1.12-2.86) using a <100 mmHg cutoff, and 1.50 (95% CI 1.02-2.21) using a <110 mmHg cutoff. Conclusion: While we found that TBI patients with a SBP <90 mmHg were over 2 times more likely to die, patients with an SBP <110 mmHg on ED arrival were still 1.5 times more likely to die from their injuries compared to patients without hypotension. These results suggest that establishing a higher threshold for clinically meaningful hypotension in TBI patients is warranted.