Aiming to protect marine wildlife, Obama will also direct the government to create a program to deter illegal fishing.
The executive steps come as Obama is searching for ways to leave his second-term mark on the environment despite opposition from many Republicans in Congress.
Obama was to announce the steps today in a video message to those participating in an "Our Ocean" conference that the State Department and Secretary of State John Kerry are hosting.
But the White House hasn't settled on the final boundaries for the preserve. The administration said it planned to solicit input from fishermen, scientists, politicians, experts in conservation and others before the new protections take effect.
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The White House's Council on Environmental Quality said the waters in the south-central Pacific Ocean contain "some of the most pristine tropical marine environments in the world."
Obama is also vowing to create a government program to combat black market fishing and seafood fraud, in which seafood products are mislabelled to hide their origin.
The White House said 20 per cent of the wild marine fish caught each year are part of the black market, at a cost of $23 billion to the legitimate fishing industry.
"President Obama's announcement is a historic step forward in the fight against seafood fraud and illegal fishing worldwide," said Beth Lowell of the conservation group Oceana.