Study objective: To identify predictors of undertriage among older injured Medicare beneficiaries, identify any regions in which undertriage is more likely to occur, and examine additional factors associated with undertriage at a national level. Methods: Using 2009 to 2014 Medicare claims data, we identified older adults (≥65 years) receiving a diagnosis of traumatic injury, and linked claims with trauma center designation records from the American Trauma Society. Undertriage was defined as nontrauma centers treatment with an Injury Severity Score greater than or equal to 16, consistent with the American College of Surgeons Committee on Trauma benchmark. We used multivariable logistic regression to estimate odds of undertriage by census region, adjusting for sex, race, age, Injury Severity Score, trauma center proximity, and mode of transportation. Results: Forty-six percent of severely injured patients (n=125,731) were treated at a nontrauma center. Compared with that for patients in the Midwest, adjusted odds of undertriage were 100% higher for patients in Southern states (odds ratio [OR] 2.00; 95% confidence interval [CI] 2.00 to 2.04) and 78% higher in Western states (OR 1.78; 95% CI 1.73 to 1.82). Compared with that for patients aged 65 to 69 years, odds of undertriage gradually increased in all age groups, reaching 57% for patients older than 80 years (OR 1.57; 95% CI 1.52 to 1.61). Distance to a trauma center was associated with increasing odds of undertriage, with 37% higher odds (OR 1.37; 95% CI 1.15 to 1.40) for older adults living more than 30 miles from a trauma center compared with patients living within 15 miles. Conclusion: Nearly half of older adult trauma patients are undertriaged; it increases with age and distance to care and is most common in Southern and Western states. Improvements to field triage and trauma center access for older patients are urgently needed.