Purpose The importance of treating severely injured patients in higher-level trauma centers is undisputable. However, it is uncertain whether severely injured patients that were initially transported to a lower-level trauma center (i.e., undertriage) benefit from being transferred to a higher-level trauma center. Methods This observational study included all severely injured patients (i.e., Injury Severity Score >= 16) that were initially transported to a lower-level trauma center within eight ambulance regions. The exposure of interest was whether a patient was transferred to a higher-level trauma center. Primary outcomes were 24-h and 30-day mortality. Generalized linear models including inverse probability weights for several potential confounders were constructed to evaluate the association between transfer status and mortality. Results We included 165,404 trauma patients that were transported with high priority to a trauma center, of which 3932 patients were severely injured. 1065 (27.1%) patients were transported to a lower-level trauma center of which 322 (30.2%) were transferred to a higher-level trauma center. Transferring undertriaged patients to a higher-level trauma center was significantly associated with reduced 24-h (relative risk [RR] 0.26, 95%-CI 0.10-0.68) and 30-day mortality (RR 0.65, 0.46-0.92). Similar results were observed in patients with critical injuries (24-h: RR 0.35, 0.16-0.77; 30-day: RR 0.55, 0.37-0.80) and patients with traumatic brain injury (24-h: RR 0.31, 0.11-0.83; 30-day: RR 0.66, 0.46-0.96). Conclusions A minority of the undertriaged patients are transferred to a higher-level trauma center. An inter-hospital transfer appears to be safe and may improve the survival of severely injured patients initially transported to a lower-level trauma center.