Purpose: For patients with time-critical acute coronary syndrome, reporting electrocardiogram (ECG) findings is the most importantcomponent of the treatment process. We aimed to develop and validate an automated Fast Healthcare InteroperabilityResources (FHIR)-based 12-lead ECG mobile alert system for use in an emergency department (ED). Materials and Methods: An automated FHIR-based 12-lead ECG alert system was developed in the ED of an academic tertiarycare hospital. The system was aimed at generating an alert for patients with suspected acute coronary syndrome based on interpretationby the legacy device. The alert is transmitted to physicians both via a mobile application and the patient’s electronic medicalrecord (EMR). The automated FHIR-based 12-lead ECG alert system processing interval was defined as the time from ED arrivaland 12-lead ECG capture to the time when the FHIR-based notification was transmitted. Results: During the study period, 3812 emergency visits and 1581 12-lead ECGs were recorded. The FHIR system generated 155alerts for 116 patients. The alerted patients were significantly older [mean (standard deviation): 68.1 (12.4) years vs. 59.6 (16.8)years, p<0.001], and the cardiac-related symptom rate was higher (34.5% vs. 19%, p<0.001). Among the 155 alerts, 146 (94%) weretransmitted successfully within 5 minutes. The median interval from 12-lead ECG capture to FHIR notification was 2.7 min [interquartilerange (IQR) 2.2–3.1 min] for the group with cardiac-related symptoms and 3.0 min (IQR 2.5–3.4 min) for the group withnon-cardiac-related symptoms. Conclusion An automated FHIR-based 12-lead ECG mobile alert system was successfully implemented in an ED.