The rhinos, which lived in Congo's Garamba National Park, were the last remaining northern white rhinos in the wild.
Other rhino species have fared better in recent years, according to IUCN. The number of southern white rhinos in Africa increased to 17,480 last year from 14,540 in 2005. African black rhinos increased to 4,180 in last year from 3,730 in 2005.
"They are still classed as critically endangered and face increasing threats of poaching and civil unrest," said Richard Emslie, a rhino specialist working for IUCN. "There is no room for complacency."
Rhinos are hunted by poachers for their horns, which are prized as trophies and as ingredients in traditional medicine.