Introduction: To our knowledge, the diagnostic value of the sP-Selectin level in the diagnosis of COVID-19 disease has not yet been investigated. In this study, we aimed to assess this by evaluating the relationship between sPSelectin level and the clinical severity of COVID-19 infections. Methods: A total of 80 patients (50 with mild to moderate and 30 with severe COVID-19 pneumonia), and 60 nonsymptomatic healthy volunteers participated in the study. Following serum isolation, sP-Selectin levels were assessed by Enzyme-Linked Immunosorbent Assay (ELISA) method. Results: The serum sP-Selectin level was 1.7 ng/ml in the control group (1-3.78); 6.24 ng/ml (5.14-7.23) in mildto-moderate pneumonia group; and 6.72 ng/ml (5.36-8.03) in the severe pneumonia group. Serum sP-Selectin levels in both mild-to-moderate pneumonia and severe pneumonia groups were found to be higher than the control group, with statistical significance (p = 0.0001 and p = 0.0001, respectively). Receiver operating characteristic analysis (ROC) showed greater area under the curve (AUC) for the serum sPSelectin levels of the COVID-19 patients (AUC = 0.913, 95% CI = 0.857-0.969; p = 0.0001). The serum sPSelectin level was found to be 97.5% sensitive and 80% specific at 4.125 ng/ml level for diagnosis (p = 0.0001). The serum sP-Selectin level was found to be 76.9% sensitive and 51.9% specific at the level of 6.12 ng/ ml (p = 0.005) to predict the need for intensive care treatment. Conclusion: This study showed that sP-Selectin can be used as a valuable biomarker in both diagnosing and predicting the need for intensive care treatment of COVID-19 infection.