South-central Florida's latest Hemphillian Palmetto Fauna includes two machairodontine felids, the lion-sized Machairodus coloradensis and a smaller, jaguar-sized species, initially referred to Megantereon hesperus based on a single, relatively incomplete mandible. This made the latter the oldest record of Megantereon, suggesting a New World origin of the genus. Subsequent workers variously accepted or rejected this identification and biogeographic scenario. Fortunately, new material, which preserves previously unknown characters, is now known for the smaller taxon. The most parsimonious results of a phylogenetic analysis using 37 cranio-mandibular characters from 13 taxa place it in the Smilodontini, like the original study; however, as the sister-taxon to Megantereon and Smilodon. Accordingly, we formally describe Rhizosmilodon fiteae gen. et sp. nov. Rhizosmilodon, Megantereon, and Smilodon ( = Smilodontini) share synapomorphies relative to their sister-taxon Machairodontini: serrations smaller and restricted to canines; offset of P3 with P4 and p4 with m1; complete verticalization of mandibular symphysis; m1 shortened and robust with widest point anterior to notch; and extreme posterior "lean" to p3/p4. Rhizosmilodon has small anterior and posterior accessory cusps on p4, a relatively large lower canine, and small, non-procumbent lower incisors; all more primitive states than in Megantereon and Smilodon. The former also differs from Megantereon and Smilodon gracilis by having a very small mandibular flange. Rhizosmilodon is the oldest known member of the Smilodontini, suggesting that the tribe originated in North America. Two more derived, similar-sized species evolved in parallel during the Blancan, Megantereon hesperus and Smilodon gracilis. The former is rarer, known only from the north-central and northwestern US, and presumably dispersed into the Old World. The latter is known from the eastern and southern US, and dispersed into South America.