The biologically rich chain of coral reefs and islands stretches from north of Hawaii to Midway Atoll.
The area, known now as North-Western Hawaiian Islands Coral Reef Ecosystem Reserve, is more than 2,253 km long and encompasses roughly 358,400 sq km - slightly bigger than Australia's Great Barrier Reef Marine Park's 348,000 sq km.
Larger than all of America's national parks combined, the new reserve is an aquatic Eden of tropical fish, sea turtles, monk seals and more than 7,000 other marine species.
At least a quarter of the species cannot be found anywhere else. All commercial fishing will be phased out in the region over the next five years.
Bush wins rare green support
Marine biologists and environmental groups, many of whom have clashed bitterly with Mr Bush over his support for new oil drilling and myriad other areas where they say he has weakened environmental protections, offered him rare praise today.
"The president is creating the world's largest marine protected area. It's as important as the establishment of Yellowstone," said Fred Krupp, president of Environmental Defence, an environmental group based in New York City.
"This is the best thing the Bush administration has done over the last six years for the oceans and for the environment," added Ellen Athas, director of ecosystems protection for the Ocean Conservancy and a former Clinton administration official.
The National Oceanic and Atmospheric Administration already had been holding public hearings to consider a national marine sanctuary in the area, an uninhabited chain of tiny islands and coral outcroppings in United States territorial waters.
Midway Atoll, one of the outermost points of the new monument, will retain an emergency landing strip for commercial and military trans-Pacific flights.
Joshua Reichert, who heads the private Pew Charitable Trusts' environment program which pushed for the sanctuary for eight years, said the region contains almost 70 per cent of the tropical shallow water coral reefs in the United States.
"When you add it all up, it's a world-class ecological jewel," he said.
Mr Bush's interest in the area was heightened after seeing a TV documentary show this year.
Mr Bush and his wife Laura watched Voyage to Kure, the first episode of Jean-Michel Cousteau: Ocean Adventures.
Mr Cousteau is the son of famed ocean explorer Jacques Cousteau and was at the White House that day. Kure is an island in the northern Hawaiian chain.