Achillesaurus is a very interesting theropod dinosaur genus, whose fossil remains were found in 1995 by a team of the Museo Argentino de Ciencias Naturales Bernardino Rivadavia of Argentina, in the Bajo de la Carpa Formation, which is found in the Rio Negro Province of Argentina. The team recovered a partial postcranial skeleton, that is, parts of its skeleton, like some of its tail vertebrae, were found but the skull was missing.

Achillesaurus is a type of alvarezsaurid theropod dinosaur that existed during the Santonian period in the Upper Cretaceous era. The name "Achillesaurus" draws inspiration from Achilles' heel, as this region holds key diagnostic features for these creatures. Achillesaurus was initially categorized as a basal alvarezsaur that thrived in the late Cretaceous period in Argentina. Nevertheless, there is currently some debate surrounding its validity.

Achillesaurus was originally believed to have coexisted with the alvarezsauridae's type genus, Alvarezsaurus. However, a study conducted in 2012 by scientists have proposed the possibility that Achillesaurus and Alvarezsaurus might, in fact, be the same dinosaur. If this hypothesis holds true, then Achillesaurus would be considered a synonym for Alvarezsaurus.