Background The Cipto Triage Method (CTM) is a method developed in Indonesia for assessing the clinical acuity of emergency department patients. To evaluate the accuracy of the CTM, this study compared triage decisions using the CTM performed by nurses to the triage decisions performed by emergency medical officers (EMO). Methods This cross-sectional diagnostic study was performed in a single-centre referral hospital in Jakarta, Indonesia. Adult patients who attended the emergency department (ED) met the inclusion criteria. Trauma cases, pregnancy, and psychiatric disorders were excluded. All eligible patients underwent triage assessment based on CTM by a triage nurse and were then referred to the ED treatment zone for triage assessment performed by an EMO. The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and likelihood ratio were calculated. Results Between April and July 2022, 902 participants were randomly selected. The overall accuracy of the triage assessment of the index test was 84.7%, and the PPV of resuscitation and urgent triage was very good (90.5% and 84.5%, respectively). In the non-urgent category, the PPV was acceptable (72%), but the NPV was very good (92.2%). Conclusions Nurse-based triage assessment using CTM in the ED had very good accuracy in determining clinical acuity.