The discovery was made after scientists were led to remains by a 98-year-old woman, who said she was aware of the bones since her childhood.
Scientists were taken to the bones of the 30 feet giant, by Filomena Avila, also called Dona Tica, after convincing her they were not fossil thieves.
"Dona Tica believed we were fossil traffickers and, at first, she lied to us, saying that there are not any bones here," said Sebastian Apesteguia, leader of the paleontologists who made the discovery.
Eventually, she led them to the site in the semi-desert Patagonian steppes, where they recovered enough bones to reconstitute 70% of the animal's skeleton.
The Bonita Mountains are located
in the south of the country
Christened 'bonitasaura salgadoi', after the Bonita Mountains near the discovery site, the dinosaur is a sauropod, a plant-eater related to the brontosaurus, and lived 83 million years ago, in the Late Cretaceous period.
It was classified in the titanosaurids family, all of which have long necks, long tails and small heads.
Its unique characteristic is the sharp ridge which runs behind its teeth which allows it to severe tree branches without damaging its frontal teeth.
The scientists hypothesise that the bonitasaura, the adults of which reached 20 tonnes, lived in herds to protect themselves from predators in the region such as the carnotaurus.
The existence of the fossil beds in the general area was known since the 1922 expedition of geologist Walter Shiller and paleontologist Santiago Roth.
But the precise location was lost and remained unknown except to Dona Tica, who as a young girl had assisted the early expedition.