Acute aortic syndromes (AAS) are cardiovascular emergencies with unmet diagnostic needs. Copeptin is released upon stress conditions and is approved for rule-out of myocardial infarction (MI). As MI and AAS share presenting symptoms, stress mechanisms and necessity for rapid diagnosis, copeptin appears as an attractive biomarker also for AAS. We thus performed a diagnostic and observational study in Emergency Department (ED) outpatients. Inclusion criteria were chest/abdominal/back pain, syncope and/or perfusion deficit, plus AAS in differential diagnosis. Blood samples were obtained in the ED. 313 patients were analyzed and 105 (33.5%) were diagnosed with AAS. Median copeptin was 38.91 pmol/L (interquartile range, IQR, 16.33-173.4) in AAS and 7.51 pmol/L (IQR 3.58-15.08) in alternative diagnoses (P < 0.001). Copeptin (≥10 pmol/L) had a sensitivity of 80.8% (95% confidence interval, CI, 72.2-87.2) and a specificity of 63.6% (CI 56.9-69.9) for AAS. Within 6 hours, the sensitivity and specificity were 88.7% (CI 79.3-94.2) and 52.4% (CI 42.9-61.8) respectively. Combination with D-dimer did not increase the diagnostic yield. Furthermore, copeptin ≥25 pmol/L predicted mortality in patients with alternative diagnoses but not with AAS. In conclusion, copeptin increases in most patients with AAS within the first hours, but the accuracy of copeptin for diagnosis AAS is suboptimal.