The botanical description of Mimosa Merrytensis Marchiori is carried out. It is a new species for science that was found by the author on rocky cliff slopes near the Ibirocai river in the southeast of the State of Rio Grande do Sul. The morphological characters clearly indicate the classification of the plant in the section Eumimosa and Bentham's Obstrigosae series . The described species is compared with the others of the same botanical series, occurring in southern Brazil, Uruguay and Argentina.
