A novel bunyavirus, SFTS bunyavirus (SFTSV), was identified as the cause of severe fever with thrombocytopenia syndrome (SFTS) in China. The virus was isolated from patients presenting with fever, thrombocytopenia, leukocytopenia, and multiorgan dysfunction. RNA sequencing revealed that SFTSV belongs to the phlebovirus genus in the Bunyaviridae family. Electron microscopy confirmed the virus's morphology, and serological tests showed a strong immune response in patients. The virus was detected in 171 patients across six provinces, with a 12% mortality rate. SFTSV was likely transmitted through ticks, particularly Haemaphysalis longicornis, and there was no evidence of human-to-human transmission. The study highlights the importance of enhanced surveillance in identifying new pathogens. The findings suggest that SFTSV is a previously unknown severe febrile disease, and further research is needed to understand its epidemiology and transmission. The study was supported by various grants from Chinese and international organizations.A novel bunyavirus, SFTS bunyavirus (SFTSV), was identified as the cause of severe fever with thrombocytopenia syndrome (SFTS) in China. The virus was isolated from patients presenting with fever, thrombocytopenia, leukocytopenia, and multiorgan dysfunction. RNA sequencing revealed that SFTSV belongs to the phlebovirus genus in the Bunyaviridae family. Electron microscopy confirmed the virus's morphology, and serological tests showed a strong immune response in patients. The virus was detected in 171 patients across six provinces, with a 12% mortality rate. SFTSV was likely transmitted through ticks, particularly Haemaphysalis longicornis, and there was no evidence of human-to-human transmission. The study highlights the importance of enhanced surveillance in identifying new pathogens. The findings suggest that SFTSV is a previously unknown severe febrile disease, and further research is needed to understand its epidemiology and transmission. The study was supported by various grants from Chinese and international organizations.