Nyasasaurus parringtoni would have been alive 10 to 15 million years before any previously known dinosaurs - and more than 150 million years before the mighty Tyrannosaurus Rex.
The size of a Labrador and slight of build, Nyasasaurus had a five foot-long tail and likely walked upright on two legs, the 'Daily Mail' reported.
With only a few ribs and arm bones to go on, the scientists can't be sure what the world's oldest dinosaur ate.
However, it is likely it had a similar diet to other early dinosaurs of small prey, insects and plants.
The fossilised bones were collected during a Cambridge University expedition to Tanzania in the 1930s and gradually examined over the decades by Natural History Museum palaeontologist Alan Charig.
He requested that the work be continued on his death and the full details have now been published.
"We don't know if it walked on two legs or four legs but our prediction is that it's two legs and the reason is that the majority of early dinosaurs are two-legged animals," Dr Paul Barrett, a NHM dinosaur expert who took part in the study, said.
"We don't know what it ate because we don't have any of the teeth of skull.
But other early dinosaurs had a mixed diet," Barrett said.
The remains do, however, allow for its size to be estimated. Nyasasaurus is believed to have been 6.5 to 10ft from nose to tail, with its tail accounting for half of that.
Its slight build means it would have lived 'in the shadow' of other much larger reptiles alive at the time.
"If the newly named Nyasasaurus parringtoni is not the earliest dinosaur, then it is the closest relative found so far," University of Washington researcher Dr Sterling Nesbitt said.
"Nyasasaurus establishes that dinosaurs likely evolved earlier than previously expected and refutes the idea that dinosaur diversity burst onto the scene in the Late Triassic, a burst of diversification unseen in any other groups at that time," Nesbitt said.
The specimen, which is detailed in the Royal Society Journal Biology Letters, is stored in London at the Natural History Museum.
Remains of a second of the creatures are kept in Cape Town.
"The bone tissue of Nyasasaurus is exactly what we would expect for an animal at this position on the dinosaur family tree," said biologist Sarah Werning, of the University of California, Berkeley, who did the bone analysis.
"It is a very good example of a transitional fossil; the bone tissue shows Nyasasaurus grew about as fast as other primitive dinosaurs, but not as fast as later ones," Werning said.