There are concerns about the accuracy of Emergency Severity Index (ESI) triage system in the geriatric population. This study was performed to compare the correlation of ESI triage with injury severity score (ISS) between adults with trauma younger than 60 years and those 60 years of age and older and to determine the ability of ESI to predict an ISS of greater than 15 in these two age groups. This was an observational study performed in an academic trauma center in Kerman, Iran. A convenience sample of trauma patients older than 16 years was included. Five-level ESI triage was performed by nurses with 2-10 years of exclusive experience in triage. The ISS scores were calculated by the researchers. Both numerical and categorical (ISS >15) forms of the score were considered as outcomes. Ultimately, a total of 556 patients were enrolled in the study. No difference in undertriage was seen between the age groups (p = 0. 51). Spearman's correlation coefficient between ESI level and ISS was −0.69 and −0.77 in patients younger than 60 years and those aged 60 years or older, respectively (z = 1.20). The areas under the curves (AUCs) for prediction of ISS of greater than 15 were also similar between the two age groups (0.89 in less than 60 and 0.85 in 60 or more). In conclusion, the performance of ESI was similar between the two age groups. Therefore, the application of the ESI triage system for initial categorization of trauma patients seems to be a reliable and easy-to-learn method for the triage of elderly as well as younger age groups.