The primates, named Plecturocebus parecis, were first discovered by biologists more than 100 years ago but have only recently been recognised as a distinct group.
The species lives on the Parecis plateau in Rondonia, Brazil, and has survived deforestation because the plateau’s steep sides make it difficult to access.
Biologists originally thought the monkeys were part of the “ashy black titis” species when they were discovered in 1914.
However, Adrian Barnett, a biologist who worked on confirming the new species, told the New Scientist that a report by the researcher who first discovered the monkeys showed he was “clearly in doubt” about their classification.
The Parecis monkeys, which are known as “otôhô” by local indigenous people, are distinguished from the dark grey ashy black titis by their chestnut brown backs and large white patches on their chests.
When the research team compared DNA from the Parecis monkeys with 10 other species, including the ashy black titis, they found it was distinctly different.
Researchers have argued the new species of monkeys should be considered “near threatened” under the criteria set out by the IUCN Red List for endangered species.
“The range is small and the population restricted,” Mr Barnett told the New Scientist.
He added that the Parecis monkey was the third new species found in Brazil this year and the 20th monkey species discovered there since 2000.
Mariluce Messias, a biologist who worked on the research project after coming across the new species in 2011, was also involved in the discovery of the Munduruku marmoset in the southeastern Amazon earlier this year.
Mr Barnett said the new discoveries may be due to deforestation making access to parts of the Amazon easier.
“One thing about deforestation is that it gives everyone access to remote areas, so sometimes scientists get to areas that have never been properly explored just before the chainsaws,” he said.
The study, titled “New species of Plecturocebus from Rondônia, Brazil“, was published in the journal Primate Conservation.