Romney is aiming to replace retiring Sen.
Romney, who has become a flag carrier for a more mainstream GOP after the party was taken over by supporters of President Trump, has always been rumored to be interested in the seat that will be vacated by Orrin Hatch.
Some Utah Republicans may still question whether the one-time abortion-rights supporter is too much of an outsider or too moderate for their tastes, but he's not expected to face any serious primary or general election challenge.
Romney's national name recognition and political clout-thanks to his 2012 presidential bid and tenure as MA governor-mean he would likely have outsize influence for a first-time senator.
Romney made the announcement in a video he posted to Twitter, reintroducing himself to voters more than five years after losing the 2012 presidential campaign to now-former President Barack Obama. Yet, following Trump's election win, Romney said he was "impressed" with Trump's transition and cabinet nominees.
"Dishonesty is Donald Trump's hallmark", Romney added.
Mitt Romney makes it official: He's running for Utah Senate seat
Mr Romney had always been expected to reveal plans to run for office, but a planned Thursday announcement was postponed in the wake of the Florida school shooting "out of respect for the victims and their families".
The video doesn't address Trump directly but does make a heavy-handed reference to immigration.
"I will owe the Senate seat to no one but the people of Utah", he said.
"Utah welcomes legal immigrants from across the world", he said. "Washington send immigrants a message of exclusion".
House Speaker Paul Ryan, R-Wis., Romney's 2012 running mate, praised Romney's candidacy, saying the campaign has his "unwavering support".
Hatch, 83, retires as the most senior Senate Republican, having served in the Senate since 1976. "His talent and drive as a public servant will ensure Utah maintains a strong seat at the table".