Probainognathus (Greek: "progressive jaw") is a genus of meat-eating mammal-like reptile (Therapsids) that lived during the lower Upper Triassic of South America. This creature had an incipient squamosal-dentary jaw-cranium joint, which is a clearly mammalian anatomical feature. It was at the very least closely related to the family of Chiniquodontidae, in which some authors have included it. Some broadly similar teeth from Europe were described under the name of Lepagia.
Known from about three dozen specimens, this creature was only about 10 cm long. Two skulls, including the holotype, were stolen from the University of La Rioja, Argentina, in February 1994. A pair of Probelesodon holotypes, (P. lewisi and P. minor), accompanied them.