WASHINGTON — President Obama is set to vastly expand a marine sanctuary northwest of the main Hawaiian Islands, White House officials said Thursday, creating the world’s largest protected marine area as he seeks to cement his environmental legacy in his last months in office.

Mr. Obama will travel next week to Midway Atoll, a remote spit of land within the Papahanaumokuakea Marine National Monument, to recognize the designation and highlight the importance of protecting pristine lands and waters as the perils of climate change intensify.

The move, which will more than quadruple the size of the refuge and has been championed by conservationists, scientists and native Hawaiians, is the latest example of Mr. Obama’s expansive exercise of executive power to preserve public lands and waters.

“This act — to build resilience in our oceans, and sustain the diversity and productivity of sea life — could usher in a new century of conservation for our most special, and fragile, ocean areas,” said Sarah Chasis, director of the oceans program at the Natural Resources Defense Council.