The Russian government has announced a 1,000 square mile protected reserve to safeguard habitat for rare Amur leopards and Amur tigers.
The national park, dubbed Land of the Leopard, is in Russia's Far East.
The Wildlife Conservation Society (WCS) says it combines existing wildlife refuges and previously unprotected land along the Chinese border.
But one expert said it was "too little, way too late" to save the Critically Endangered leopard.
Conservation groups have commended Russia for the move.
WCS Russia's program director Dale Miquelle said he was "optimistic" that it would "provide a critical refuge for some of the most endangered big cats on the planet".
An estimated 30 Amur leopards occupy a narrow sliver of forest between the Sea of Japan to the east and Jilin Province, China, to the west.
In a wider area of forest habitat, about 500 Amur tigers are thought to remain in the wild.
Joerns Fickel, from the Institute for Zoo and Wildlife Research in Berlin, Germany, who has studied both the Amur leopard and Amur tiger said it was a "significant step in the right direction" but pointed out that safeguarding such a large area from poachers would be difficult.
"Regarding the tigers, he explained that prey density and cover were the "two main components to secure [their] survival".
"[It is not clear whether] this has been considered and if the park size provides space for at least a moderate population growth.
"For the leopards - where we don't even know how many of the last ones are still reproducing - but the situation is very gloomy."
The WCS pointed out that tigers regularly moved across the border into China, and the reserves' proximity to that border represented "a critical source population for recovery of tigers in North-East China".
WWF agreed that this "connection across the border" was crucial.
The conservation organisation's head of species, Diane Walkington, said: "Hopefully, by providing leopards with protected space to live, hunt and breed they will start to make a similar recovery to the Amur tiger, which is still endangered but has made a spectacular comeback since the 1930s when as few as 20 were left."
Peter Zahler, WCS deputy director for Asia said in a statement: "The creation of this park greatly increases the amount of land protecting critical populations of two of the world's big cats, and it will go a long way to securing their future.
"We look forward to continuing to provide whatever support is requested to help conserve tigers and leopards in the region."
Dr Fickel was less optimistic.
"It's better than nothing, and maybe for the tiger, it is not totally too late,"
"[But] for the leopards, hope dies last."