This study is a taxonomic treatment of Mimosa from the state of Rio de Janeiro, a significant region of the Atlantic Domain in southeastern Brazil. We recognize 36 species (38 taxa), corresponding to 31% of the Mimosa species listed for the domain. A total of 816 specimens were analyzed, and around 40% of the studied taxa are endemic to the Atlantic Domain. Dense ombrophilous forests, seasonal semideciduous forests, plus secondary vegetation types have the highest species richness, despite the genus being found in all vegetation types in the state. Five species are endangered in Brazil. An identification key, morphological descriptions and comments, a table summarizing vegetation data, illustrations, and geographic distributionmaps are provided. This study extends the known geographic distribution of several taxa of Mimosa, andincludesnewrecords for the study area. Additionally, we propose the synonymization of M. extensa Benth. var. annae Barneby with the typical variety.