Multivariate floristic analyses of Butia yatay palm groves and gallery forest associated with the Uruguay River basin of Entre Rios Province (Argentina) were performed, including vegetation stands of Rio Grande do Sul State (Brazil). Several new phytosociological associations were identified. These include Eugenio myrcianthis-Butietum yatay association, which represents palm groves on sandy soils evolved from ancient river deposits, and Bignonio callistegioidis-Terminalietum australis association, which represents some gallery forests on regular floodplains of the Uruguay River. Both are placed in the new Pampean alliance Guettardo uruguensis-Bution yatay, which is part of the Dyckio brevifoliae-Terminalietalia australis order and Sebastianio schottiae-Terminalietea australis class, which has been described for Brazil.