Proadinotherium was characterized by its teeth with a lower crown, less hypsodont than those of Adinotherium, but evocating more derived toxodontids.

Its dentition was complete with a complex structure, and the development of a crest on the molars.

Proadinotherium is considered to be the most basal and oldest member of the Toxodontidae, the most specialized group of the notoungulates, which included the well known Pleistocene genus Toxodon, as well as a number of Miocene and Pliocene forms.

The genus Proadinotherium was first described in 1894 by Florentino Ameghino, based on fossil remains found in Argentine Patagonia, with the type species being Proadinotherium leptognathum, known from various remains from the Santa Cruz Province and Chubut Province.

Ameghino described several other species, from more recent Early Miocene deposits of Patagonia, such as P. angustidens and P. muensteri.