The real estate mogul, reality television star and on-again-off-again presidential candidate initially had been thought to be backing Romney rival Newt Gingrich.
"Mitt is tough, he's smart, he's sharp, he's not going to allow bad things to continue to happen to this country that we all love," Trump said. "So, Governor Romney, go out and get 'em. You can do it."
"There are some things that you just can't imagine happening in your life," Romney said as he shared the podium with Trump and wife, Ann, at Trump International in Las Vegas. "This is one of 'em."
Trump said he would not mount an independent candidacy if Romney is the Republican candidate.
"Our country is in serious trouble," Trump said.
The endorsement from the billionaire businessman, who sharply criticized Romney earlier in the campaign, came just a day after Romney committed a political gaffe, saying in a CNN interview he doesn't worry about the very poor because they have a government safety net.
It was unclear whether the Trump endorsement would make a difference in Saturday's Nevada caucuses.
Gingrich, a former U.S. House speaker, told reporters in Reno, Nev., Wednesday he was unaware of Trump's plans.
Romney created a stir Wednesday when he told CNN he was "not concerned about the very poor."
"I'm in this race because I care about Americans," he told CNN in an interview. "I'm not concerned about the very poor. We have a safety net there. If it needs repair, I'll fix it. I'm not concerned about the very rich, they're doing just fine. I'm concerned about the very heart of America, the 90 percent, 95 percent of Americans right now who are struggling and I'll continue to take that message across the nation."
Romney, whose net worth is estimated at more than $200 million, cited food stamps, housing vouchers and other benefits in explaining his comment on concern for the poor.
Gingrich quickly picked up on Romney's comment.
"I am fed up with politicians of either party dividing Americans against each other," he said. "I am running to be the president of all the American people, and I am concerned about all the American people."
Romney told ABC News he expected the campaign against Obama would be the bitterest and nastiest in U.S. history.
"No question that Barack Obama's billion-dollar machine will organize the most vitriolic, spiteful campaign in American history," he said. "We have to be ready for that."
Romney began receiving Secret Service protection Thursday.