Earthjustice, which filed on behalf of ten groups including the Wilderness Society and the Natural Resources Defense Council, challenges the reduction of the Grand Staircase-Escalante Monument, according to AP.
"This is the largest elimination of protected land in American history", it said, directing visitors to support groups fighting to preserve public lands.
But tribal groups called Trump's action a "shameful attack" on lands considered sacred to American Indians.
The battle over Trump's authority to shrink monuments may be just beginning.
The five tribes have said they will bring a legal case against the administration - the outcome could redefine the president's powers to use the Antiquities Act.
"To see those eroded away after the antiquities act that passed in 1906 by Teddy Roosevelt, who was a republican by the way". Bears Ears features thousands of Native American artifacts, including ancient cliff dwellings and petroglyphs.
Here in Utah, where about 2/3 of the entire state is federally owned and there are seven large monuments, this Act is a household name, and in some rural areas, a dirty word.
Unlike national parks, which are established by Congress, the Antiquities Act allows national monuments to be designated either by Congress or the president.
The two national monuments are among 27 that Trump ordered Interior Secretary Ryan Zinke to review earlier this year.
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Tribes and conservationists have been preparing lawsuits for months.
The tribes argue that federal law only gives presidents the ability to create a national monument, not the ability to downsize one.
The tribes point to a federal lands law from the 1970s that says only Congress can actually reduce or nullify a national monument.
The California-based company, which is known for its activism, joins a growing number of organizations questioning the Trump administration's authority to slash the areas covered by Utah's Grand Staircase-Escalante National Monument and Bears Ears National Monument.
But there's another tricky issue here.
Patagonia also released materials and information today that center around one message: "The President Stole Your Land".
That's partly why there was so much opposition to Bears Ears, which originally was 1.3 million acres in size. The President's move was met with protests in Utah and in San Francisco by environmentalists and some Native Americans. He's ecstatic that President Trump is trying to shrink monuments across the rural West. "They don't know your land". "He understands what rural communities are about".
Zinke is also proposing Trump create three monuments: at Camp Nelson, an 1863 Union Army site in Kentucky; the Jackson, Mississippi home of civil rights leader Medgar Evers; and the Badger II Medicine Area, a 130,000-acre section of the Lewis and Clark National Forest in his home state of Montana.