Abstract Background Coronavirus disease 2019 (COVID-19) pneumonitis associated with severe respiratory failure is associated with high mortality. The pathogenesis of COVID-19 is associated with microembolism or microvascular endothelial injuries. Here, we report that syndecan-1 (SDC-1), a component of the endothelial glycocalyx, may be a biomarker of severity classification for COVID-19 related to endothelial injury. Methods and analysis We analyzed the data of COVID-19 patients for 1 year from February 2020 at Yokohama City University Hospital and Yokohama City University Medical Center Hospital. We selected COVID-19 patients who required admission care, including intensive care, and analyzed the classification of severe and critical COVID-19 retrospectively, using various clinical data and laboratory data with SDC-1 by ELISA. Results We analyzed clinical and laboratory data with SDC-1 in five severe COVID-19 and ten critical COVID-19 patients. In the two groups, their backgrounds were almost the same. In laboratory data, the LDH, CHE, and CRP levels showed significant differences in each group (P = 0.032, P