This study describes the identification and characterization of a novel bunyavirus, designated SFTS bunyavirus (SFTSV), associated with severe fever with thrombocytopenia syndrome (SFTS) in China. Since 2009, heightened surveillance in rural areas of Hubei and Henan provinces led to the discovery of SFTS, characterized by fever, thrombocytopenia, leukocytopenia, and multi-organ dysfunction. The virus was isolated from patients' blood samples using various cell lines, and its RNA was detected through polymerase-chain-reaction (PCR) assays. Electron microscopy revealed virions with bunyavirus characteristics. Serologic assays confirmed specific immune responses in all 35 pairs of serum samples collected during the acute and convalescent phases of the illness. SFTSV was detected in 171 patients from six provinces, with 21 deaths reported. The virus is likely transmitted by *Haemaphysalis longicornis* ticks, which are widely distributed in the Asia-Pacific region. The findings suggest that SFTS is caused by this newly identified bunyavirus, highlighting the importance of enhanced surveillance in identifying emerging infectious diseases.This study describes the identification and characterization of a novel bunyavirus, designated SFTS bunyavirus (SFTSV), associated with severe fever with thrombocytopenia syndrome (SFTS) in China. Since 2009, heightened surveillance in rural areas of Hubei and Henan provinces led to the discovery of SFTS, characterized by fever, thrombocytopenia, leukocytopenia, and multi-organ dysfunction. The virus was isolated from patients' blood samples using various cell lines, and its RNA was detected through polymerase-chain-reaction (PCR) assays. Electron microscopy revealed virions with bunyavirus characteristics. Serologic assays confirmed specific immune responses in all 35 pairs of serum samples collected during the acute and convalescent phases of the illness. SFTSV was detected in 171 patients from six provinces, with 21 deaths reported. The virus is likely transmitted by *Haemaphysalis longicornis* ticks, which are widely distributed in the Asia-Pacific region. The findings suggest that SFTS is caused by this newly identified bunyavirus, highlighting the importance of enhanced surveillance in identifying emerging infectious diseases.