The landscape of canyons, ridges and rock formations would be split into three zones.
While a handful of monuments have been resized in the past, none has been cut back to such an extent, putting the president's proclamation in uncharted legal territory.
The Navajo Nation has already said it plans to challenge Mr Trump’s action in court.
But Mr Trump blasted the move as having “severely abused” the Antiquities Act, saying in a speech that Mr Obama had empowered “faraway bureaucrats at the expense of the people who actually live here, work here, and make this place their home”.
In an appearance at Utah’s state Capitol in Salt Lake City, the President confirmed his decision to reduce Bears Ears National Monument from around 1.5 million acres to some 228,000 acres and to cut the Grand Staircase Escalante National Monument to one million acres, about half its current size.
It marked Mr Trump’s latest effort to unravel the legacy of his predecessor, Barack Obama, who in 2016 invoked the Antiquities Act to designate Bears Ears a national monument.
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