The new species were found in a remote region known as the "Green Corridor" in Thua Thien Hue province in central Vietnam, it said.
"You only discover so many new species in very special places, and the Green Corridor is one of them," Chris Dickinson, WWF's chief technical adviser in the region, said in a statement.
The new snake species, the white-lipped keelback, generally lives close to streams and eats frogs and other small animals, WWF said. It has a yellow-white stripe along its head, red dots over its body and can reach a length of 80 centimeters (31.5 inches).
The new butterfly species are among eight discovered in Thua Thien Hue since 1996. One is a "skipper," a butterfly that flies in a quick, darting motion. It is from the genus Zela. The other is from a new genus in the subfamily Satyrinae.
Three of the new orchid species are leafless, which is unusual for orchids, WWF said.
The other new plant species include one in the aspidistra family, which produces a black flower and can subsist in low light, and an arum, which produces yellow flowers surrounded by funnel-shaped leaves, it said.
"It's great news for Vietnam," said Bernard O'Callaghan, Vietnam program coordinator for the World Conservation Union. "The jungles and mountains of Vietnam are fascinating places and they continue to surprise scientists."
All the new species are exclusive to tropical forests in Vietnam's Annamites mountain range, which offers unique habitats.
All species in the area are under threat from illegal logging, hunting and development.
Many threatened species live in the Green Corridor, including the white-cheeked crested gibbon, one of the world's most endangered primates.