Atypical lymphocytes in the peripheral blood in patients with severe fever with thrombocytopenia syndrome (SFTS) have not been well examined. In this study, we analyzed counts and characteristics of atypical lymphocytes in 7 patients with SFTS. Atypical lymphocytes resembled plasma cells morphologically and appeared in the peripheral blood of all patients 4–8 days after onset of disease. Among these lymphocytes flow cytometry showed a CD19⁺CD38⁺CD138^−/+ phenotype, and immunohistochemical staining revealed a CD79a⁺CD38⁺CD138^−/+CD27⁺ phenotype. From our observations, atypical lymphocytes transiently that appeared in the peripheral blood during the acute phase of SFTS were considered to be plasmablasts.