The president tweeted, "He will make a great Senator and worthy successor to @OrrinHatch and has my full support and endorsement!"
Romney, who accepted Trump’s endorsement, announced his intention to run last week, but never mentioned Trump in a campaign video.
Romney bashed Trump during the 2016 election, including a March 2016 speech in which he meticulously outlined what he described as a minefield of problems Trump presented if elected. The former Massachusetts governor ripped Trump over foreign policy, his businesses and the economy, his temperament and dealings with Russia.
"Donald Trump is a phony, a fraud," Romney told an audience at the University of Utah last March. "His promises are as worthless as a degree from Trump University. He's playing the American public for suckers: He gets a free ride to the White House and all we get is a lousy hat."
Trump, in turn, said Romney "choked like a dog" when he lost in 2012 to President Obama and called him a "mixed up man who doesn't have a clue."
But Romney accepted Trump's endorsement on Twitter late Monday, making no mention of his previous critical comments.
He’s on the same page policy-wise as Trump, Romney said in an interview with The Associated Press on the day of his campaign announcement Friday, though adding he wouldn't hesitate to speak out against the president if he disagreed with him. He mentioned an immigration plan similar to Trump's in an interview with the The Salt Lake Tribune, offering support for "a border fence or wall" and saying he agreed that chain migration and the lottery program should be fixed.
ABC News' Meridith McGraw contributed to this report.