The Soterion Rapid Triage System is a new, computerized, five-level triage acuity system. The purpose of this study was to evaluate the system's inter-rater reliability and validity for stratifying patient acuity. The study was comprised of two components. A prospective evaluation of inter-rater reliability was conducted by the blinded, paired simultaneous triage of 423 patients. A retrospective evaluation of validity consisted of the analysis of 33,850 patients triaged with the system over an 8-month period. The system's validity was measured by in-hospital admission rate, Emergency Department (ED) length-of-stay, hospital charges and Current Procedural Terminology (CPT) Codes 99281-99285. Evaluation of inter-rater reliability demonstrated a weighted kappa of 0.87 (95% confidence interval 0.84, 0.91). The in-hospital admission rates for patients triaged as Level 1 Immediate-Level 5 Non-Urgent were 43%, 30%, 13%, 3% and 1%, respectively (p < 0.0001). Similarly, there were significant differences in the means for all hospital charges, ED lengths-of-stay, and CPT Codes. In conclusion, the Soterion Rapid Triage System possesses high inter-rater reliability and validity. The system's reliability and validity, and the availability of the system's electronically archived data are characteristics beneficial to the development of a national standardized five-level triage acuity system.