Mesozoic mammals from Gondwana remain poorly understood, with most species known only from isolated teeth. Nevertheless, these tantalizing fossils point to a diversity of families, that were distinct from Laurasian forms. Among these, the enigmatic South American family Reigitheriidae, was proposed based on the unique crushing teeth of its sole representative, Reigitherium from Northern Patagonia. Here, we report a new mammal, Yeutherium pressor gen. et sp. nov., which is only the second known taxon of Reigitheriidae. This is based on a partial maxilla bearing an upper molar from the Late Cretaceous Dorotea Formation in the Río de Las Chinas Valley, Sub-Antarctic Chile. Parsimony and Bayesian phylogenetic analyses indicate a close relationship between Reigitherium and Yeutherium. Shared features between both taxa—such as expanded pre- and postparacristae, a lingual cingulum at the paracone base, interradicular crests, and enamel crenulations—help define Reigitheriidae. Cingula development and molar morphology of Yeutherium differ from that of the related Mesungulatidae, suggesting independent evolution of the crushing adaptations in Reigitheriidae. Additionally, we suggest a reinterpretation of the dental formula of Reigitheriidae. The discovery of Yeutherium highlights the specialised dentition of Reigitheriidae as a significant functional innovation in a radiation of Gondwanan mammals that is starting to be understood.