This is the first study to report the frequencies and activation status of circulating dendritic cells (DCs) in patients at different stages of severe fever with thrombocytopenia syndrome (SFTS). The decreased percentage of DCs in peripheral blood mononuclear cells (PBMCs) was correlated with the severity of SFTS. The circulating level of myeloid dendritic cells (mDCs) can be used as a valuable prognostic biomarker for SFTS patients. These findings lead to a better understanding of the mechanisms underlying the progression of SFTS.