The conservation group The Gorilla Organization said the last observed birth of mountain gorilla twins was in 2004, the BBC reported Wednesday.
"It is very rare to hear of mountain gorilla twins, so we were all excited to hear the news from rangers ...," Emmanuel Bugingo, the group's program manager in Rwanda, said of Thursday's births.
Fewer than 800 mountain gorillas are thought to live in the wild, though their numbers are believed to be increasing.
"Gorilla mothers usually have only one baby every four years or so -- which is one reason why they are so vulnerable -- so twins give a rare double gain in one birth," says Ian Redmond of the Ape Alliance, an international coalition that works for the welfare of apes.
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'