Populations of naturalized mimosa (Albizia julibrissin Durazz., Fabaceae) are reported from 32 counties in the southern third of Illinois. Literature and field studies suggest a significant range expansion of this taxon within Illinois during the last 50 yr. Most populations occur in open areas or along roadsides and forest edges, but a high-quality loess hill prairie on the Mississippi River bluffs contains high densities of mimosa seedlings and saplings. Mimosa is expected to continue to spread into suitable habitat based on its long history of invasion.