The White House is to close a loophole that allows thousands of firearms to be sold at gun shows and on the internet without federal background checks.
It will classify around 23,000 vendors as licensed firearms dealers, making their gun sales subject to the checks.
Attorney General Merrick Garland said: "If you sell guns predominantly to earn a profit, you must be licensed."
The Biden administration said it expected a court challenge by pro-gun groups, but was confident it would win.
"All of the major actions that the president has taken to reduce gun violence have been challenged," an administration official said.
"And in court after court, the actions are frequently being upheld. We have confidence that this is legal."
The National Rifle Association (NRA) said it was preparing to fight the new regulations, and Republican senators have also objected.
Nevertheless,Mr Garland said that the regulation was a "historic step" that would "save lives". He pointed to federal data showing that illegal or black-market firearms are increasingly being found at shooting scenes.
Vice-President Kamala Harris, who leads the Office on Gun Violence Prevention, said unlicensed gun dealers had "gotten away" with unverified sales for far too long.
In a video on Thursday, President Joe Biden called on Congress to "finish the job and pass universal background check legislation now".
The closure of the "gun show loophole" will go into effect 30 days after it is published in the federal registry this week.
The White House has cited the Bipartisan Safer Communities Act of 2022, which solidified who and what is considered a firearm dealer, to expand the background check requirements.
The law was passed with support from Republicans and Democrats after two high-profile mass shootings in May 2022 - an attack on a supermarket in Buffalo, New York, and a school shooting in Uvalde, Texas.
However, a spokesman for John Cornyn, a prominent Republican senator who worked on the Bipartisan Safe Communities Act, called the move "unconstitutional" .
Joined by Senator Thom Tillis, a fellow Republican who helped negotiate the bill's passage, Mr Cornyn plans to introduce a joint resolution of disapproval under the Congressional Review Act.
The pair claim the new regulation attempts to "rewrite the law" and will "go against congressional intent", according to a copy of their proposed resolution, which was reviewed by CBS News, the BBC's US partner.
Randy Kozuch, the executive director of the NRA's legal branch, said the move was an "attack on law-abiding gun owners" and that he was "working to use all means available to stop this unlawful rule".
Around 80,000 licensed firearms dealers in the US are already required to conduct a background check.
There were more than 40,000 gun-related deaths in the United States last year, according to the Gun Violence Archive.
BBC