A new species of Adianthus is described from the Rio Cisnes (type locality of the Friasian age), Miocene of Chile. The species is represented by unusually complete remains, including the first postcranial elements known for a member of the family Adianthidae. In its skeletal anatomy, Adianthus godoyi, new species generally resembles lightly-built Santa-crucian proterotheriids. The new species is unique among litopterns in having the proximal tibia and fibula solidly fused. Like Santacrucian proterotheres, Adianthus godoyi was probably cursorially adapted; the narrowness of the anterior dental arcade suggest that it was a selective-feeding herbivore, and perhaps consumed mixed vegetation in an open habitat. Adianthus godoyi appears to be closely related and to an as yet unidentified species from the early Santacrucian (Notohippus fauna) of Argentina and to Adianthus bucatus, from the Santacrucian of that country, which otherwise represents the latest known occurrence of the family Adianthidae. The occurrence of Adianthus godoyi in the type fauna of the Friasian thus suggests either that the family was more long-lived than had been previously appreciated, or that the type Friasian local fauna is more similar to those of Santacrucian age than their placement in different land-mammal ages would suggest.