Undertriage may lead to serious outcomes such as mortality, morbidity, and medical litigations. This study aimed to determine the rate, outcomes, and predictive factors of undertriage among red zone patients at Hospital Sultan Abdul Halim, Kedah. A retrospective study was conducted from January 2018 until December 2019 involving 393 patients, with 131 and 262 patients categorized as undertriaged and appropriately triaged, respectively. Undertriage patients were identified as patients who fulfilled the red zone category based on the Malaysian Triage Category but were mistriaged to green. The numerator was the number of red zone patients who were mistriaged to green, and the denominator consisted of all red zone patients. Patient characteristics (demography, comorbidities, chief complaints, vital signs, Glasgow Coma Scale), environmental factors (working shift, weekend, public holiday), and staff factors (paramedic and nurses’ working experience) were obtained from electronic medical records and proforma forms. Logistic regression was employed to analyse the data. The rate of undertriage was 1.2% (131/10,867 cases), slightly higher than the 0.5% set by the Health Ministry of Malaysia. The predictors of undertriage were comorbidity of asthma (AOR= 3.791; p=0.008), Glasgow Coma Scale (AOR=2.110; p=0.002), and patients with specific chief complaints of cardiovascular (AOR=0.189; p value <0.001), respiratory (AOR = 0.262; p value<0.0001), neurological (AOR = 0.081; p value<0.0001) and trauma (AOR = 0.129; p value<0.0001). Asthmatic and higher GCS patients were more likely to be undertriaged, while patients with specific chief complaints were less likely to be undertriaged.