Liarthrus ("smooth joint")[1] is a genus of astrapotheriid mammal known from the Late Oligocene (Deseadan SALMA, around 29–21 mya) Sarmiento Formation of Santa Cruz Province, Argentina.

[2][1] It was described by the Argentine paleontologist Florentino Ameghino in 1895 along with several other genera from the "Pyrotherium Beds", which were then believed to date to the Cretaceous period.

[3] Ameghino described Liarthus on the basis of fragmentary, being only a right astragalus (tarsal bone), premolar 4, and an incomplete premolar from the upper jaws.

[4][2] Only one species was described, L. copei, the species name honoring the American paleontologist Edward Drinker Cope, who lived during the same interval as Ameghino.

[2] Liarthrus was a herbivorous mammal, being an astrapothere, which had large tusks on their skulls and mandibles in addition to a large body size.