US President George W Bush has designated a swathe of Hawaiian islands as a US national monument, making them the world's largest marine sanctuary.
Fishing will be phased out over five years
He signed a law on Thursday which will give the Northwestern Hawaiian Islands the highest protected status in US law.
The area, nearly as big as California, supports more than 7,000 species, a quarter of which occur nowhere else.
Environmental groups welcomed the decision, although fishing industry bodies have raised concerns.
The designated site - more than 140,000 sq miles (362,000 sq km) of reefs, atolls and shallow seas - is just larger than the Great Barrier Reef Marine Park in Australia, previously the world's largest protected marine area.
The remote and uninhabited islands and surrounding seas are important breeding grounds for sea turtles, and are home to the only remaining population of the endangered Hawaiian monk seal.
The new restrictions will mean all fishing is phased out within five years and visitors will need permits to snorkel or dive in the area.
The islands were already being considered for designation as a national marine sanctuary.
Now, however, Mr Bush has used his powers under the 1906 National Antiquities Act, which allows the president to give instant protection to important sites, in a decision which will bypass a year-long process of consultation and afford a greater level of protection.
"This is really for the first time saying the primary purpose of this area of the ocean is to be a pristine, or nearly pristine, kind of place," David Festa, director the ocean programme at Environmental Defense, told the New York Times.
"It would take it off the books as a fishing ground. That's really the first time we'll have done that in any kind of sizeable area," he said.
In an interview with the Washington Post, local Democrat representative Ed Case lauded the president for undertaking "the most revolutionary act by any president, any administration, in terms of marine resources".
Although only eight fishing boats are licensed to fish in the area, a local fisheries body says it plans to fight a complete ban on fishing.
"We supported the sanctuary concept but wanted the continuation of our healthy bottom fisheries up there," Kittie Simonds, of the Western Pacific Fishery Management Council, told the New York Times.
According to the Pew Charitable Trusts, lobster populations in the area have not recovered from aggressive exploitation in the 1980s and 1990s, although this is now banned.
Recent research also shows signs of over-fishing in the islands' remaining fisheries, the organisation says.
Although environmental groups welcomed the news, many remain strongly opposed to other Republican policies on the environment, including a push to open up the Arctic National Wildlife Refuge in Alaska to oil and gas exploration.