Introduction The American College of Surgeons (ACS) delineates trauma team activation (TTA) criteria to identify seriously injured trauma patients in the field. Patients are deemed to be severely undertriaged (SU), placing them at risk for adverse outcomes, when they do not meet TTA criteria but nonetheless sustain significant injuries (Injury Severity Score ISS ≥25). Objectives Delineate patient demographics, injuries, and outcomes after SU. Participants Trauma patients presenting to our ACS-verified Level 1 trauma center with ISS ≥25 were included (11/2015-03/2022). Transfers and private vehicle transports were excluded. Patients were dichotomized and compared by trauma arrival level: TTA (Appropriately Triaged, AT) vs routine consults (SU). Results Study criteria were satisfied by 1653 patients: 1375 (83%) AT and 278 (17%) SU. Severely undertriaged patients were older than AT patients (47 vs 36 years, P < .001). Severely undertriaged occurred almost exclusively following blunt trauma (96% vs 71%, P < .001). Injury Severity Score was lower following SU than AT (29 vs 32, P < .001). The most common severe injuries (Abbreviated Injury Scale score AIS ≥3) among the SU group were in the Chest (n = 179, 64%). Severely undertriaged patients necessitated emergent intubation (n = 34, 12%), surgery (n = 59, 21%), and angioembolization (n = 22, 8%) at high rates. Severely undertriaged mortality was n = 40, 14%. Conclusion Severely undertriaged occurred among a substantial proportion of ISS ≥25 patients, predominately following blunt trauma. Severe chest injuries were most likely to evade capture. Rates of intubation, emergent intervention, and in-hospital mortality were high after SU. Efforts should be made to identify such patients in the field as they may benefit from TTA.