Local adaptation to climate has facilitated the global invasion of cheatgrass.

Local adaptation may facilitate range expansion during invasions, but the mechanisms underlying successful invasions remain unclear. Cheatgrass (Bromus tectorum), native to Eurasia and Africa, has invaded globally, with severe impacts in western North America. We aim to identify mechanisms and consequences of local adaptation in the North American cheatgrass invasion. We sequence 307 range-wide genotypes and conduct controlled experiments. We find that diverse lineages invaded North America, where long-distance gene flow is common. Nearly half of North American cheatgrass comprises a mosaic of ~19 locally adapted, near-clonal genotypes, each seemingly very successful in a different part of North America. Additionally, ancestry, phenotype, and allele frequency-environment clines in the native range predict those in the invaded range, indicating pre-adapted genotypes colonized different regions. Common gardens show directional selection on flowering time that reverse between warm and cold sites, potentially maintaining clines. In the USA Great Basin, genomic predictions of strong local adaptation identify sites where cheatgrass is most dominant. Our results indicate that multiple introductions and migration within the invaded range fuel local adaptation and success of cheatgrass in western North America. Understanding how environment and gene flow shape adaptation and invasion is critical for managing ongoing invasions.