Returns from 10 percent of the state's precincts showed Romney with 51 percent of the vote, to 28 percent for Gingrich.
Former Pennsylvania Sen. Rick Santorum had 12 percent, and Texas Rep. Ron Paul 7 percent. Neither Santorum nor Paul mounted a substantial effort in the state.
The winner-take-all primary was worth 50 Republican National Convention delegates.
But the bigger prize was precious political momentum in the race to pick an opponent for Democratic President Barack Obama this fall. That belonged to Romney when he captured the New Hampshire primary three weeks ago, then swung stunningly to Gingrich when he countered with a South Carolina upset 11 days later.
About half of Florida primary voters said the most important factor for them was backing a candidate who can defeat Obama in November, according to early exit poll results conducted for The Associated Press and the television networks.
As in early contests in Iowa, New Hampshire and South Carolina, that mattered more than experience, moral character or conservative credentials.
Not surprisingly, in a state with an unemployment rate hovering around 10 percent, about two-thirds of voters said the economy was their top issue. More than 8 in 10 said they were falling behind or just keeping up. And half said that home foreclosures have been a major problem in their communities.
Gingrich swept into Florida from South Carolina, only to run headlong into a different Romney from the one he had left in his wake in South Carolina.
Romney, the former Massachusetts governor, shed his reluctance to attack Gingrich, the former House speaker, unleashing hard-hitting ads on television, sharpening his performance in a pair of debates and deploying surrogates to the edges of Gingrich's own campaign appearances, all in hopes of unnerving him.
Restore our Future, an outside group supporting Romney, accounted for about $8.8 million in the ad wars, and the candidate and the "super PAC" combined outspent Gingrich and Winning The Future, the organization backing him, by about $15.5 million to $3.3 million, an advantage of nearly 5-1.
Gingrich responded by assailing Romney as a man incapable of telling the truth and vowed to remain in the race until the Republican National Convention next summer. He won the endorsement of campaign dropout Herman Cain and increasingly sought the support of evangelicals and tea party advocates, a former House speaker running as the anti-establishment insurgent of the party he once helped lead.
Bombarded by harsh television advertising, some Floridians said they had soured on both candidates.
"The dirty ads really turned me off on Mitt Romney," said Dorothy Anderson, of Pinellas Park, adding she was voting for Gingrich. She said of Romney, "In fact if he gets the nomination, I probably won't vote for him."
At the same polling place, Romney supporter Curtis Dempsey expressed similar feelings but about Gingrich. "The only thing Newt Gingrich has to offer is a big mouth," he said.
Voters frequently say they are offended or appalled by negative ads. But polls show consistently that the commercials are able to sway the opinions of large numbers of voters, and they are a staple of nearly all campaigns.
Santorum had no money for television ads to back up his strong debate performances. He left the state at one point, saying he was going home to Pennsylvania to prepare his income tax returns. But he stayed longer than anticipated, because of the hospitalization of his 3-year-old daughter with pneumonia. The girl has a rare genetic disorder, Trisomy 18.
Santorum and Paul both campaigned in Colorado on Tuesday as Florida Republicans were voting. The state has caucuses on Feb 7, the same day as Minnesota.
Even before that come caucuses in Nevada, a state that Romney won when he sought the nomination in 2008 and is favored to capture again.
By contrast, both Romney and Gingrich campaigned across Florida on primary day as the polls opened.
Exuding confidence, the former Massachusetts governor said -- even though the figures said otherwise -- that he had been outspent in South Carolina.
"I needed to make sure that instead of being outgunned in terms of attacks, that I responded aggressively, and hopefully that will have served me well here," he told reporters.
Gingrich, combative as usual, said the race for the nomination won't be decided until summer, "unless Romney drops out."
More than 600,000 Floridians voted before the polls opened, either by absentee or early ballot, exceeding the figure from four years ago, and raising the possibility of a record turnout. Vote totals in the three contests to date -- Iowa, New Hampshire and South Carolina -- have all been records.