They include a monkey with ghostly white circles around its eyes, frogs, newts and the only known succulent bamboo species, a WWF report says.
Some of the world's most endangered species in the area face extinction even before they have been identified.
The Mekong region includes Cambodia, Laos, Myanmar, Thailand and Vietnam.
"These species are extraordinary, beautiful products of millions of years of evolution," K. Yoganand, WWF-Greater Mekong's regional lead for wildlife and wildlife crime, was quoted as saying by Reuters.
The officials added that the species were "under intense threat", with many of them "going extinct even before they are described".
Risks to the species in the richly biodiverse area in South-East Asia include habitat destruction, the introduction of diseases by humans and the illegal trade in wildlife, the WWF says.