BACKGROUND: Secondary triage is a critical mechanism through which severely injured patients presenting to lower-resource hospitals are transferred to high-resource trauma centers and is associated with improved survival. We conducted a contemporary, nationally representative analysis to quantify the volume of severely injured patients not transferred to higher-level care and to identify predictors of nontransfer. STUDY DESIGN: This is retrospective cohort study using the 2019 Nationwide Emergency Department Sample of all adult trauma patients with an Injury Severity Score greater than 15 who first presented to a Level III or nontrauma center (NTC). The primary outcome was nontransfer, defined as admission to the Level III or NTC from the emergency department. Multivariable generalized linear models were developed to determine patient- and hospital-level predictors of nontransfer. RESULTS: Among 146,816 encounters, 84,695 patients (58%) were not transferred, reflecting secondary undertriage. Independent patient predictors of nontransfer included increasing age (80 years or older; adjusted odds ratio [aOR] 1.68, 95% CI 1.43 to 1.97) and public insurance (Medicare aOR 1.76 [95% CI 1.54 to 2.02] and Medicaid aOR 1.44 [95% CI 1.27 to 1.65]). Hospital-level predictors included Level III trauma designation (aOR 2.93, 95% CI 2.10 to 4.08) and metropolitan location (aOR 5.21, 95% CI 3.43 to 7.92). These predictors persisted in the subanalysis of patients with an Injury Severity Score of 25 or more. CONCLUSIONS: One in 3 severely injured trauma patients in the US are first treated at Level III or NTCs, of whom more than half are not transferred to higher-level trauma centers. Hospital factors including metropolitan location and Level III designation were the strongest predictors of nontransfer. These findings support the development of inclusive state trauma systems that incorporate all acute care hospitals, including NTCs, in trauma registries and regional quality improvement initiatives.