For many years, it was believed that there were just five species of sakis, but this study showed the existence of 16 - five of them new to science.
The five new species are found in Brazil, Peru, and Bolivia - three of them are endemic to Brazil and one to Peru.
Three species recognised were previously considered to be subspecies, and another three, already described, were previously thought to be just variants.
The study is the largest revision for any Neotropical primate genus in more than half a century, researchers said.
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"I began to suspect there might be more species of saki monkeys when I was doing field research in Ecuador," Marsh said.
"The more I saw, the more I realised that scientists had been confused in their evaluation of the diversity of sakis for over two centuries," said Marsh.
"Saki monkeys, like many rain forest primates, are excellent indicators for the health of tropical forest systems," said Russell A Mittermeier, President of Conservation International, after whom one of the new species, Pithecia mittermeieri, is named.
Saki monkeys are distributed throughout the Amazon Basin and the Guiana Shield. They are elusive and little studied in the wild, and our understanding of their conservation status is minimal, researchers said.
The research was published in the journal Neotropical Primates.