Objective To comprehend the time of emergence and extent of cryptic circulation for Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV) in Thailand. Methods Plasma specimens collected from patients at Siriraj Hospital, Bangkok, Thailand presenting with acute undifferentiated febrile illness (AUFI) were characterized by NGS. Molecular and serological diagnostics were developed to screen for viral RNA and antibodies. Phylogenetic and phylogeographic analyses were performed using sequences generated from this study, publicly available genomes, and unsampled taxa to enhance temporal and geographic resolution. Results NGS detected SFTSV in 7 individuals with a median age of 73 years. Clinical manifestations ranged from low-grade fever and altered consciousness to multi-organ failure and death. RT-qPCR revealed three additional RNA positives and antibody screening identified 38 IgG-positives for an incidence and prevalence of 0.4% and 2.7%, respectively. While SFTSV is reported as introduced into Thailand in 2019, evidence of infections dates back to 2012. Phylogenetic analyses revealed multiple introductions of Lineage 2 into Thailand around 2011-2012 and phylogeographic reconstructions identified Thailand as a source for SFTSV spread to China and neighboring countries. Conclusions SFTSV has circulated cryptically in Thailand since 2012, diversifying locally and establishing endemicity. Genomic surveillance through improved diagnostics will be necessary to curb its spread through Asia and beyond.