Abstract: The global invasion, and subsequent spread and evolution of weeds provides unique opportunities to address fundamental questions in evolutionary and invasion ecology. Amaranthus palmeri is a widespread glyphosate-resistant (GR) weed in the USA. Since 2015, GR populations of A. palmeri have been confirmed in South America, raising questions about introduction pathways and the importance of pre- versus post-invasion evolution of GR traits. We used RAD-Seq genotyping to characterize genetic structure of populations from Brazil, Argentina, Uruguay and the USA.