A fossil angiosperm wood from the Touro Passo Formation (Pleistocene) in the northwestern part of Rio Grande do Sul State, Brazil, is described as the new fossil-taxon Itaquixylon heterogenum Benicio, Pires, Da Rosa, Spiekermann, Uhl et Jasper gen. nov. et sp. nov. The fossil wood is silicified, and is 9.9 cm in diameter and 16.6 cm in length, with small pith. The main anatomical features are diffuse porosity, vessels solitary and in radial multiples, simple perforation plates, alternate vestured intervessel pits, vessel-ray and vessel-parenchyma pits bordered, axial parenchyma scanty paratracheal and vasicentric, heterocellular rays, non-septate fibers, extremely short and with simple pits, as well as distinct growth rings. The unique set of anatomical features allows the proposing of a new fossil-genus with an affinity to the subfamily Mimosoideae of the Fabaceae. This record extends the area from which fossil woods are known in the western portion of Rio Grande do Sul State. The systematic classification proposed here, adds a new fossil-taxon related to the currently diverse family Fabaceae, thus contributing to knowledge of past biodiversity on the basis of wood anatomy in this family during the Pleistocene of Southern Brazil.