The find was announced by Conservation International, a group that helped finance the research in remote rain forests that until recently were considered too dangerous for scientific work due to the presence of leftist rebels.
A team of researchers from the National University of Colombia observed 13 groups of the new species - dubbed the Caqueta titi monkey because it was found in the southern state of Caqueta, near Peru.
The researchers, who published their findings in the journal Primate Conservation, believe the species may be critically endangered. They estimate less than 250 of the monkeys exist and say the felling of forest for agriculture threatens their habitat.
The new variety of titi monkey, which has the scientific name Callicebus caquetensis, is the size of a cat and has grayish-brown hair. What sets it apart from other types of titi monkey species is its lack of a white bar on the forehead.
"It's a spectacular finding," said Jeffrey French, a biology and psychology professor at the University of Nebraska at Omaha who works with primates in the Amazon in Brazil.
Some formerly rebel-held regions of Colombia have become safer in recent years due to the government's gains against guerrilla bands, facilitating efforts to search for oil and precious metals as well as flora and fauna.
The research team, including professors Thomas Defler and Marta Bueno and student Javier Garcia, visited Caqueta in 2008 - three decades after Martin Moynihan, an animal behavior expert, first caught sight of the species in the area. Insecurity in the area prevented research to confirm his sighting until the team arrived.
The researchers say the monkeys are monogamous - unlike most primates but common among titi monkeys - and often have one baby a year. They have complex calls and were spotted often moving around in groups of four.
Juan Mayer, a former Colombian environment minister, said that due to deforestation, "huge efforts will have to be made to protect the creature's habitat."
Paul Garber, a primate researcher and anthropology professor at the University of Illinois, said titi monkeys play important roles in the forest, dispersing seeds, pollinating plants and helping the forest regenerate.
He said the new variety of monkey is important because it shows there are probably other undiscovered primate species in the world's tropical forests.
"We need to provide resources to aid scientists so that these species can be identified, studied and protected," Garber said.