The Aardonyx celestae was a small-headed herbivore with huge barrel of a chest, and the scientists said it could prove to be a missing evolutionary link.
"This is definitely without a shadow of a doubt a brand new type of dinosaur - one that no one has seen before and one that has a very significant position in the family tree of dinosaurs," said Australian paleontologist Adam Yates.
Yates, who is based at the University of the Witwatersrand's Bernard Price Institute for Paleontological Research, led the research with a number of other local and international scientists.
Their findings were published Wednesday in the Proceedings of The Royal Society B, a London-based peer-reviewed journal.
The Aardonyx celestae walked on its hind legs but could drop to all fours and stood nearly 6 feet (about 1.7 meters) high at the hip, the scientists said. It was about 10 years old when it died and weighed about 1,100 pounds (500 kilograms). Its death may have been caused by drought.
Yates says the Aardonyx celestae has many characteristic of the plant-eating herbivores that walked on two legs as well as their descendants. Those sauropods, known more popularly as brontosaurs, grew to massive sizes and went about on all fours.
The newly discovered species shows an intermediary stage in the evolution of these creatures, Yates said.
"Aardonyx gives us a glimpse into what the first steps toward becoming a sauropod involved," he said.
The discovery of the new species was made by postgraduate student Marc Blackbeard, who was excavating two sites about five years ago.
Yates believes that the scientists may have stumbled onto a "paleontological oasis" in central South Africa that may yield further previously unknown dinosaur species.
Already they are working on the bones of two other specimens found at the site and have also unearthed some giant teeth believed to belong to a mysterious carnivorous dinosaur.