The reverse shock index (rSI), a ratio of systolic blood pressure (SBP) to heart rate (HR), is used to identify prognosis in trauma patients. Multiplying rSI by Glasgow Coma Scale (rSIG) can possibly predict better in-hospital mortality in patients with trauma. However, rSIG has never been used to evaluate the mortality risk in adult severe trauma patients (Injury Severity Score [ISS] ≥ 16) with head injury (head Abbreviated Injury Scale [AIS] ≥ 2) in the emergency department (ED). This retrospective case control study recruited adult severe trauma patients (ISS ≥ 16) with head injury (head AIS ≥ 2) who presented to the ED of two major trauma centers between January 01, 2014 and May 31, 2017. Demographic data, vital signs, ISS scores, injury mechanisms, laboratory data, managements, and outcomes were included for the analysis. Logistic regression and receiver operating characteristic analysis were used to evaluate the accuracy of rSIG score in predicting in-hospital mortality. In total, 438 patients (mean age: 56.48 years; 68.5% were males) were included in this study. In-hospital mortality occurred in 24.7% patients. The median (interquartile range) ISS score was 20 (17-26). Patients with rSIG ≤ 14 had seven-fold increased risks of mortality than those without rSIG ≤ 14 (odds ratio: 7.64; 95% confidence interval: 4.69-12.42). Hosmer-Lemeshow goodness-of-fit test and area under the curve values for rSIG score were 0.29 and 0.76, respectively. The sensitivity, specificity, positive predictive value, and negative predictive values of rSIG ≤ 14 were 0.71, 0.75, 0.49, and 0.89, respectively. The rSIG score is a prompt and simple tool to predict in-hospital mortality among adult severe trauma patients with head injury.