Background: Pulmonary thromboembolism (PTE) is a serious and lifethreatening condition. Diagnosis of PTE can be challenging in emergency departments, as there is no absolutely reliable biomarker for the diagnosis of PTE. Copeptin (COP) is a new biomarker, which may be valuable in the diagnosis of PTE; however, its role has not been well studied. In this study, we aimed to investigate the diagnostic value of COP in the diagnosis of PTE. Materials and Methods: This study was carried out on 102 patients suspected of PTE. The serum levels of D-dimer and COP were measured, and diagnosis of PTE was confirmed by CT angiography. Next, the prognostic value of D-dimer and COP was examined. Results: The area under the curve (AUC) of D-dimer was 0.581 with a standard error (SE) of 0.07 (P=0.34). Estimation of the validity of D-dimer showed that it is a highly sensitive (100%), but poorly specific (15.8%) test. Evaluation of the predictive value of this test showed that it has a positive predictive value of 20% and a negative predictive value of 100%. The AUC of COP was 0.423 with SE of 0.1 (P=0.44). Measurement of the validity of COP test showed that it is a poorly sensitive (50%) and specific (22.9%) test. Conclusion: COP is a new cardiovascular biomarker. However, the present findings did not confirm the prognostic value of this biomarker for the diagnosis of PTE.