The subspecies of the black rhino -- which is classified as "critically endangered" by the International Union for Conservation of Nature's (IUCN) Red List of Threatened Species -- was last seen in western Africa in 2006.
The IUCN warns that other rhinos could follow saying Africa's northern white rhino is "teetering on the brink of extinction" while Asia's Javan rhino is "making its last stand" due to continued poaching and lack of conservation.
"In the case of the western black rhino and the northern white rhino the situation could have had very different results if the suggested conservation measures had been implemented," Simon Stuart, chair of the IUCN species survival commission said in a statement.
"These measures must be strengthened now, specifically managing habitats in order to improve performance, preventing other rhinos from fading into extinction," Stuart added.
The IUCN points to conservation efforts which have paid off for the southern white rhino subspecies which have seen populations rise from less than 100 at the end of the 19th century to an estimated wild population of 20,000 today.
Another success can be seen with the Przewalski's Horse which was listed as "extinct in the wild" in 1996 but now, thanks to a captive breeding program, has an estimated population of 300.
The latest update to the IUCN Red List of Threatened Species reviews more than 60,000 species, concluding that 25% of mammals on the list are at risk of extinction.
"We have the knowledge that conservation works if executed in a timely manner, yet, without strong political will in combination with targeted efforts and resources, the wonders of nature and the services it provides can be lost forever."