A colourless cavefish and an iridescent snake are among the species discovered in the area that is home to some of the world's most endangered species.
Scientists have discovered more than 200 new species across the greater Mekong region in 2020, despite the threats posed by climate change and human activities such as logging.
In all, 224 new species of plants and vertebrate animals were found in the region - which includes Myanmar, Thailand, Laos, Cambodia and Vietnam - World Wildlife Fund (WWF) said in its "New Species Discoveries" report.
The finds include a new primate, a colourless cavefish and an iridescent snake with an unusual non-overlapping pattern of scales.
Images of the Popa langur monkey, which takes its name from the extinct volcano Mount Popa in central Myanmar, were caught by camera traps.
The mountain is still home to the largest population of the reclusive simian, around 100 individuals, WWF said.
Only around 200 to 250 of the monkeys - which are threatened by hunting, logging and loss of habitat - are thought to survive in total.
In Vietnam, researchers found the vivid-coloured Mount Ky Quan San horned frog at an altitude of more than 2,000 metres (6,500 feet) on the peak which gives it its name.
WWF have said the rate of discovery of new species - more than 3,000 since 1997 - shows the importance of preserving the region's fragile ecosystems.