The discovery of 15 partial skeletons is the largest single discovery of its type in Africa.
Researchers say it will change ideas about human ancestry.
Fossils of the creature were unearthed in a deep cave near the famed sites of Sterkfontein and Swartkrans, treasure troves 50 kilometres northwest of Johannesburg that have yielded pieces of the puzzle of human evolution for decades.
The fact the fossils were found in a chamber has led paleoanthropologists to believe that the ancient species also appeared to bury its dead, a trait previously believed to be uniquely humanm, through a process of deduction.
Lee Berger, a research professor in human evolution at the University of Witwatersrand, led the expedition.
He introduced the species "Homo naledi" to the world.
He has told the BBC the creatures have been classified in the grouping Homo, to which modern humans belong.
"Naledi is a fantastic new species, part of our family tree. Where it fits is something that's going to be the next phase of study. What we know about it is quite a lot right now. We know about it from 1,550 elements so far, but we have thousands left in the Dinaledi chamber where we discovered it."
Professor Berger says Homo naledi also possess human-like features.
"It's a tall, primitive human ancestor. (It) would have stood about five feet tall, (had) a small brain about the size of a large orange, but a very human-like character to the overall plan of the skull. It's got small teeth like yours, but primitive in their shape and form, which lets us know that it's somewhere in that transitional stage of an early human, an early member of our genus, but not quite to where we are yet."
But he says some parts of the creature's body came as a surprise.
"It's got an ape-like shoulder complex, an ape-like core -- a primitive core within our lineage that's something more like the most primative hominins like Lucy or Mrs Ples, right down to the pelvis. When you move down below, though, and you go down below the leg, it has long legs, which culminate in this incredibly human-like foot, something that was a complete surprise."
Professor Berger adds that the species' arms and hands were also human-like.
"If you pop up back to the top and look at the arms, they're more and more human-like as you move away from that primitive shoulder, until you reach the wrist, which is incredibly human-like, the palm, which is human-like. The whole plan of the hand is human-like, but the distal fingers are some of the most curved we've ever seen in the entire human-ancestor record."
South Africa's Deputy President Cyril Ramaphosa has welcomed the discovery.
"For us to understand how this species lived right here in South Africa, right here on the African continent, is something that is a great, great step for us. And one could echo what was once said, that this could well be a small step for naledi -- naledi took a small step into that chamber -- but for us, as the people of the world, this is a gigantic step to understand who we are."
It is not the first time the study of humans' relatives, extinct or living, has yielded evidence humans have no monopoly on certain kinds of behaviour.
In 1960, Jane Goodall famously observed chimpanzees, humans' closest living relatives, using grass stems for termite "fishing," the first recorded use of a crude tool by non-humans.
Homo naledi, discovered in the cave in September 2013, had a brain slightly larger than a chimpanzee's, but its age remains an enigma.
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