Background: Severe fever with thrombocytopenia syndrome (SFTS) exhibits marked geographic heterogeneity across East Asia. We aimed to investigate the evolutionary drivers and ecological determinants underlying the decoupling of SFTS incidence and clinical severity between inland and coastal regions.<br><br>Methods: We integrated 1,820 high-quality global genomes of Dabie Bandavirus (SFTSV) with clinical data from 936 patients and 27,457 reported cases in China. Generalized additive models identified meteorological drivers, while Bayesian phylogeographic inference quantified maritime transmission. Multivariable logistic regression and evolutionary pressure analysis identified genotype-specific mutations assfociated with fatal outcomes.<br><br>Findings: An “Inland-High-Incidence vs. Coastal-High-Fatality” pattern was identified. Inland transmission is sensitive to thermal fluctuations, whereas coastal dynamics are governed by a sunshine threshold (&gt; 200 h/month). Zhejiang province serves as a genetic hub with higher recombination rates than inland regions (11·0% vs 3·5%, P &lt; 0·001). Cross-ocean transmission from Japan and South Korea shapes the high-fatality genetic architecture in coastal China. Notably, while marine barriers maintain genetically distinct clusters on adjacent islands, maritime connectivity facilitates rapid long-distance viral dispersal. In clinical, Genotypes A and C are linked to elevated mortality, with the RdRp N828S mutation identified as a robust molecular predictor of fatal outcomes.<br><br>Interpretation: Divergent environmental pressures and maritime transmission networks drive SFTSV evolution and epidemiology. These findings provide a phylogeographically informed framework for targeted surveillance and genotype-specific interventions in high-risk hotspots.​