"It's as close to the Garden of Eden as you're going to find on Earth," said Bruce Beehler, co-leader of the team.
The scientists claim to have discovered 20 frog species, four butterfly species and at least five new types of palms.
But their discoveries will have to be reviewed by peers before being officially classified as new species.
The team - from the US, Indonesia and Australia - surveyed a region near the Foja Mountains in Papua province in eastern Indonesia, which covers an area of more than a million hectares (two million acres) of forest.
"There was not a single trail, no sign of civilisation, no sign of even local communities ever having been there," Mr Beehler told the Associated Press.
He said that even two local tribesmen, who accompanied the scientists, were astonished at the area's isolation.
"As far as they knew, neither of their clans had ever been to the area," Mr Beehler said.
Unafraid of humans
One of the team's most remarkable discoveries was a honey-eater bird with a bright orange patch on its face - the first new bird species to be sighted in the area for more than 60 years.
Mr Beehler said some of the creatures the team came into contact with were remarkably unafraid of humans.
Two Long-Beaked Echidnas, primitive egg-laying mammals, even allowed scientists to pick them up and bring them back to their camp to be studied, he added.
The December 2005 expedition was organised by the US-based organisation Conservation International, together with the Indonesian Institute of Sciences.
The team admit that in their month-long trip, they did not have enough time to survey the area completely.
"We just scratched the surface," Mr Beehler told reporters. "Anyone who goes there will come back with a mystery."
Mr Beehler himself hopes to return later this year.