The World Wildlife Fund, WWF, identified 224 new species discovered in the Greater Mekong area in 2020.
The area includes Vietnam, Cambodia, Laos, Thailand and Myanmar.
Among the species is a monkey with white circles around its eyes.
The list also included numerous newly identified reptiles, frogs and newts, fish and 155 plant species.
A report by the WWF marks the need to protect rich wildlife habitats in the Mekong.
The report said the discoveries demonstrate the strength of species to survive in environments threatened by deforestation, disease and other harms.
K. Yoganand is the WWF's Mekong leader for wildlife and wildlife crime.
He told Reuters that the new species represent "beautiful products of millions of years of evolution."
Today, however, they are "under intense threat," Yoganand said, with many species disappearing before they are even described.
The newly discovered monkey is called the Popa langur.
It got its name because it lives on the hillsides of Myanmar's former Mt. Popa volcano.
A new type of begonia plant with reddish flowers and a berry-like fruit also was found in a hilly area of Myanmar.
The area has a problem with illegal mining and logging.
Scientists have now identified more than 3,000 new species in the Mekong area since 1997, the WWF said.
Scientists used measurements and samples from public collections to compare and identify important elements of the newly discovered animals and plants, the report said.
Researchers say identifying new species can be difficult and sometimes requires several different methods.
In one case, frog calls and genetic data were combined to differentiate the Cardamom leaf-litter frog.
The animal was found high in the Cardamom mountains in a protected wildlife area.
Some species can be found in more than one country, including the bright orange twin slug snake.
The Popa langur was identified based on genetic matching.
Recently gathered bones were compared with samples from Britain's Natural History Museum found more than a century ago.
The WWF worked with the group Fauna and Flora International, FFI, to capture images of the monkeys using camera traps in 2018.
FFI reported the discovery late last year.
The monkey is a candidate to be listed as a critically endangered species on the Red List of the International Union for the Conservation of Nature, the report said.
Only 200 to 250 of the monkeys are thought to survive in the wild.
I'm Bryan Lynn.