[1] The type and only species is Riograndia guaibensis, named after the State of Rio Grande do Sul and Guaíba Basin, where it was discovered.

[1][4] Holotype (MCN-PV 2264) is an anterior part of a skull, from the tip of snout to the fronto-parietal contact, with complete dentition.

The below cladogram was found by Martinelli and Rougier in 2007 and modified by Soares et al. in 2011,[3] with Tritheledontidae added after Ruta et al. (2013):[5] Brasilodon Brasilitherium Sinoconodon Morganucodon Riograndia Irajatherium Chaliminia Elliotherium Pachygenelus Diarthrognathus Tritheledon A cladogram after Stefanello et al. (2023):[6] Lumkuia Chiniquodon Probainognathidae Ecteniniidae Protheriodon Prozostrodon Pseudotherium Therioherpeton Irajatherium Riograndia Diarthrognathus Pachygenelus Tritylodontidae Botucaraitherium Brasilodon Mammaliaformes All specimens of Riograndia come from the Late Triassic aged Caturrita Formation.

Non-dinosaurian animals include the dinosauriform Sacisaurus; the dicynodont Jachaleria; an unnamed phytosaurian; isolated archosaurian teeth; an amphibian classified in Stereospondyli; and many common tetrapods smaller than 15 cm (5.9 in) long.

Among the tetrapods is the procolophonid Soturnia; the sphenodontid Clevosaurus; the lepidosaurian Cargninia; the archosaurian Faxinalipterus, and an assortment of mammaliamorphs including Riograndia, Brasilodon, and Irajatherium.