The discovery suggests there are more areas of healthy coral in the ocean's unmapped "twilight zone," the UN heritage body says.
Scientists have discovered a vast reef of "pristine" rose-shaped corals apparently unharmed by climate change in deep water off the coast of Tahiti.
Announcing the discovery on Thursday, UNESCO said it was "one of the most extensive healthy coral reefs on record".
Mapping approximately three kilometres (two miles) long and up to 65 metres (213 feet) wide, UNESCO said it was "highly unusual" to find healthy coral in cooler waters between 30 and 65 metres deep.
The agency said it could suggest that there are more reefs in the ocean depth that are safer from the impacts of warming waters.
The discovery was made in November by divers with special equipment that allowed them to go deeper and spend 200 hours at the reef.
"It was magical to witness giant, beautiful rose corals which stretch for as far as the eye can see. It was like a work of art," said Alexis Rosenfeld, a French photographer and founder of the UNESCO partner 1 Ocean campaign.
The giant rose-shaped corals are each up to two metres in diameter.
"To date, we know the surface of the moon better than the deep ocean," said UNESCO Director-General Audrey Azoulay, adding that only 20 percent of the world's seabed has been mapped.