WASHINGTON — President Obama on Thursday created the Atlantic Ocean’s first United States marine monument, preserving an expanse of sea canyons and underwater mountains off the New England coast as he races to use his executive power to protect vast stretches of land and water before he leaves office.

Mr. Obama announced the designation of the Northeast Canyons and Seamounts Marine National Monument at an ocean conservation meeting in Washington, shortly after issuing a proclamation protecting an area roughly the size of Connecticut that sits 130 miles off the coast of Cape Cod.

“If we’re going to leave our children with oceans like the ones that were left to us, then we’re going to have to act, and we’re going to have to act boldly,” Mr. Obama said at the State Department, recalling his childhood spent in Hawaii, bodysurfing in the Pacific Ocean and gazing out at its waters.

The new monument lies in an area of the Atlantic where ocean temperatures are projected to warm as much as three times as fast as the global average, and is home to endangered whales and turtles, ancient deep-sea coral and species of fish unique to the region. Mr. Obama said protecting it was a vital step to counteract increasingly grave trends taking hold as the planet warms.