Two new genera of late Pleistocene Mimosoideae from Argentina are described. The fossils were recovered from the Arroyo Feliciano Formation in the upper Río Gualeguay Basin, eastern Argentina. The anatomical features suggest a relationship with Cylicodiscus Harms for the first one and Abarema Pittier and Hydrochorea Barneby and J.W. Grimes for the second. Cylicodiscuxylon paragabunensis gen. nov. and sp. nov. is characterised by diffuse-porous wood, solitary vessels, in radial multiples and in clusters; alternate and vestured intervessel pits; vasicentric, aliform, lozenge-aliform and confluent axial parenchyma, strands of 5-11 cells, prismatic crystals in chains in chambered axial parenchyma cells; 1-3 (4) seriate rays and homocellular, vessel-ray parenchyma pits similar in size and shape to intervessel pits, and non-septate fibres. Abaremaxylon hydrochorea gen. nov. and sp. nov. has the following diagnostic features: diffuse-porous wood, solitary vessels, in radial multiples and in clusters; alternate and vestured intervessel pits; vasicentric, aliform and unilateral aliform, confluent axial parenchyma, strands of 4-16 cells; prismatic crystals in chains in chambered axial parenchyma cells; exclusively uniseriate and homocellular rays, and non-septate fibres. The fossils described herein represent the first report of legume woods from the Río Gualeguay Basin. The record of Cylicodiscuxylon in southern South America supports the existence of an old relationship with the African flora and Abaremaxylon reinforces the idea that Neotropical flora was widespread in the past. The analysis of fossil plants suggests that they grew under warm and humid to semi-arid climatic conditions.