BACKGROUND There are well-known disparities for patients injured in rural setting versus urban setting. Many cite access to care; however, the mechanisms are not defined. One potential factor is differences in field triage. Our objective was to evaluate differences in prehospital undertriage (UT) in rural setting versus urban settings. METHODS Adult patients in the Pennsylvania Trauma Outcomes Study (PTOS) registry 2000 to 2017 were included. Rural/urban setting was defined by county according to the Pennsylvania Trauma Systems Foundation. Rural/urban classification was performed for patients and centers. Undertriage was defined as patients meeting physiologic or anatomic triage criteria from the National Field Triage Guidelines who were not initially transported to a Level I or Level II trauma center. Logistic regression determined the association between UT and rural/urban setting, adjusting for transport distance and prehospital time. Models were expanded to evaluate the effect of individual triage criteria, trauma center setting, and transport mode on UT. RESULTS There were 453,112 patients included (26% rural). Undertriage was higher in rural patients (8.6% vs. 3.4%, p < 0.01). Rural setting was associated with UT after adjusting for distance and prehospital time (odds ratio [OR], 3.52; 95% confidence interval [CI], 1.82-6.78; p < 0.01). Different triage criteria were associated with UT in rural/urban settings. Rural setting was associated with UT for patients transferred to an urban center (OR, 3.32; 95% CI, 1.75-6.25; p < 0.01), but not a rural center (OR, 0.68; 95% CI, 0.08-5.53; p = 0.72). Rural setting was associated with UT for ground (OR, 5.01; 95% CI, 2.65-9.46; p < 0.01) but not air transport (OR, 1.18; 95% CI, 0.54-2.55; p = 0.68). CONCLUSION Undertriage is more common in rural settings. Specific triage criteria are associated with UT in rural settings. Lack of a rural trauma center requiring transfer to an urban center is a risk factor for UT of rural patients. Air medical transport mitigated the risk of UT in rural patients. Provider and system interventions may help reduce UT in rural settings. LEVEL OF EVIDENCE Care Management, Level IV.