Palau is to create the world's first "shark sanctuary", banning all commercial shark fishing in its waters.
The President of the tiny Pacific republic, Johnson Toribiong, announced the sanctuary during Friday's session of the UN General Assembly.
With half of the world's oceanic sharks at risk of extinction, conservationists regard the move as "game-changing".
It will protect about 600,000 sq km (230,000 sq miles) of ocean, an area about the size of France.
President Toribiong also called for a global ban on shark-finning, the practice of removing the fins at sea.
Fins are a lucrative commodity on the international market where they are bought for use in shark fin soup.
As many as 100 million sharks are killed each year around the world.
"These creatures are being slaughtered and are perhaps at the brink of extinction unless we take positive action to protect them," said President Toribiong.
"Their physical beauty and strength, in my opinion, reflects the health of the oceans; they stand out," he told BBC News from UN headquarters in New York.
The president also called for an end to bottom-trawling, a fishing method that can destroy valuable seafloor ecosystems such as coral reefs.
A number of developed nations have implemented catch limits and restrictions on shark finning.
Some developing countries such as The Maldives have also taken measures to protect the creatures; but Palau's initiative takes things to a new level, according to conservationists close to the project.
"Palau has recognised how important sharks are to healthy marine environments," said Matt Rand, director of global shark conservation at the Pew Environment Group.
"And they've decided to do what no other nation has done and declare their entire Exclusive Economic Zone a shark sanctuary.
"They are leading the world in shark conservation."
Mr Rand said that about 130 threatened species of shark frequented waters close to Palau and would be likely to gain from the initiative.
Although the country has only 20,000 inhabitants, its territory encompasses 200 scattered islands, which means that its territorial waters are much bigger than many nations a thousand times more populous.
Economics is clearly an incentive for the Palau government, which derives most of its income from tourism.
Sharks are themselves a big attraction for scuba-divers, and may also play a role in keeping coral reef ecosystems healthy.
Globally, 21% of shark species whose extinction risk has been assessed fall into the "threatened" categories, and 18% are "near threatened". For a further 35%, there is not enough data to decide.
Over half of the species that spend most of their time in the upper layers of the ocean, exposed to fishing, are on the threatened list.
Illegal shark-finning is the main cause; but there are legal targeted hunts for fins and meat, and sharks are also caught accidentally on longlines set for fish such as marlin and tuna.
Port side catches
Enforcing the ban will be an issue for Palau, which possesses just one patrol boat capable of monitoring its waters.
A recent aerial survey found fishing 70 vessels in the area, most of them illegally.
But Carl-Gustaf Lundin, who heads the marine programme at the International Union for the Conservation of Nature (IUCN), said there were other ways of tackling the illegal trade.
"For example, the US has been sharing lists of illegal vessels with established fishing companies, so that they can report on their dishonest or non-decent peers," he said.
"We're also exploring what options there are for monitoring remotely at low cost.
"And you don't need to catch people out there in the ocean; everyone needs to land their fish, so as long as you have most nations signed up to oppose illegal fishing, your chances of catching them are pretty decent."
Dr Lundin noted that earlier this week, another Pacific island state, Kiribati, signed off a collaboration with the US that establishes the largest marine reserve on the planet.
"The time for setting aside tiny areas of sea that only protect a few sedentary species is over; and it (the Palau sanctuary) is important because it shows the way in terms of putting large areas aside."
In organisations such as the International Whaling Commission and the Convention on International Trade in Endangered Species (CITES), Palau has in recent years regularly sided with pro-hunting countries such as Japan.
Mr Toribiong told BBC News that before going to New York for the UN General Assembly, he had planned to "state to the world that Palau will revisit its current position" on whaling.
But following the recent change of government in Japan, and because of the two countries' "close relationship", he said there would now be a bilateral meeting.
"My position is to reconsider our current position in light of the most recent scientific data to ensure that the current position that Palau takes will not lead to the depletion and extinction of whales," he said.
But when it came to sharks, the president said he was sure that the sanctuary is backed by science - sharks are threatened as a group of species, and sanctuaries can help.
"Not all nations consider shark fins as delicacies," he said.
"And we feel that the need to protect the sharks outweighs the need to enjoy a bowl of soup."
Asked what he would be urging other leaders to do, he said simply: "To follow suit."