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In early 2018 Nigeria experienced an unprecedented increase in Lassa fever cases with widespread geographic distribution. We report 77 Lassa virus genomes generated from patient samples, 14 from 2018, to investigate whether recent changes in the virus genome contributed to this surge. Our data argue that the surge is not attributable to a single Lassa virus variant, nor has it been sustained by human-to-human transmission. We observe extensive viral diversity structured by geography, with major rivers appearing to act as barriers to migration of the rodent reservoir. Together our results support that the 2018 Lassa fever surge was driven by crossspecies transmission from local rodent populations of multiple viral variants from different lineages.