Stylosanthes viscosa (L.) Sw. (Fabaceae) is a herbaceous legume with a native range that includes much of the Neotropics, from Argentina to Texas and several West Indian islands. Except for native populations in Texas, there are no published records of this species growing outside cultivation in the continental United States. Here, I report extensive populations of S. viscosa in southeastern Florida, often growing in near monoculture, with an observed range spanning more than 35 km from Lakewood Park to Port St Lucie in St Lucie County and to Jensen Beach in northernmost Martin County. These populations may have descended from plants imported from Brazil and grown at the Agricultural Research Center (now the Indian River Research and Education Center) in Fort Pierce, St Lucie County, where at least one field of this species growing in near monoculture persists. Stylosanthes viscosa has the potential of becoming a dominant invasive species in and adjacent to open habitats in peninsular Florida.