Thylacosmiline sparassodonts (previously recognized as thylacosmilids) are among the most iconic groups of endemic South American Cenozoic mammals due to their distinctive morphology and convergent resemblance to saber-toothed placental carnivores. However, the early evolution of this group and its relationship to other sparassodonts remains poorly understood, primarily because only highly specialized Neogene taxa such as Thylacosmilus, Anachlysictis, and Patagosmilus are well known. Here, we describe a new Paleogene sparassodont, Eomakhaira molossus, from the Cachapoal locality of central Chile, the first sparassodont reported from early Oligocene strata of the Abanico Formation. Eomakhaira shares features with both Neogene thylacosmilines and Paleogene “proborhyaenids,” and phylogenetic analyses recover this taxon as sister to the clade of Patagosmilus + Thylacosmilus. This broader clade, in turn, is nested within the group conventionally termed Proborhyaenidae. Our analyses support prior hypotheses of a close relationship between thylacosmilines and traditionally recognized proborhyaenids and provide the strongest evidence to date that thylacosmilines are proborhyaenids (i.e, the latter name as conventionally used refers to a paraphyletic group). To reflect the internestedness of these taxa, we propose use of Riggs' (1933) original name Thylacosmilinae for the less inclusive grouping and Proborhyaenidae for the more inclusive one. Saber teeth arose just once among metatherians (among thylacosmilines), perhaps reflecting a developmental constraint related to nonreplacement of canines in metatherians; hypselodonty may have relaxed this potential constraint in thylacosmilines. The occurrence of Eomakhaira in strata of early Oligocene age from the Chilean Andes demonstrates that the stratigraphic range of thylacosmilines spanned almost 30 million years, far surpassing those of saber-toothed placental lineages.