He finished just eight votes ahead of former senator Rick Santorum in the Midwest state of Iowa.
Ron Paul came third, while Newt Gingrich and Michele Bachmann settled into a second tier of candidates.
Rick Perry indicated he was suspending his campaign after finishing fifth.
The caucus meetings were the first time voters had a say in the race to face Democratic President Barack Obama in November’s presidential election.
Tuesday’s contest launched months of caucuses and primary elections in 50 states, Washington DC and other territories, culminating in the Republican National Convention in August where the party nominee will be formally anointed.
Iowa was not expected to settle the contest – John McCain, the eventual Republican nominee in 2008, came fourth in the state’s caucuses that year – but it will help shape the race for the White House.
The BBC’s Mark Mardell says that in the end, this result has to be good for Mitt Romney, achieving it in a state he hadn’t bothered to fight until the last few weeks.
Hours after caucuses closed, Iowa party chairman Matt Strawn announced that Mr Romney had won by just eight of the 122,255 votes cast.
“Governor Mitt Romney received 30,015 votes and senator Rick Santorum received 30,007 votes. Congratulations to governor Mitt Romney, winner of the 2012 caucuses. Congratulations to senator Rick Santorum for a very close second-place finish,” Mr Strawn said.
Earlier in the evening, Mr Romney kept his sights firmly trained on Mr Obama rather than engaging his Republican rivals or claiming victory.
“The gap between his promises four years ago and his performance is as great as anything I’ve ever seen in my life,” he said, before crying: “On to New Hampshire!”
As early results came out pointing to a close race, Mr Santorum declared “Game on!” He praised his faith and his family in a speech which marked his own entry to the national spotlight.
Mr Paul, a Texas congressman, finished third and vowed to continue onto New Hampshire, which holds a primary election next week.
“This momentum is going to continue,” he told a jubilant crowd of supporters. “We will go on, we will raise the money.”
Finishing fifth, Texas Governor Rick Perry said he was returning to his home state in order to “determine whether there is a path forward for myself in this race”.Former House of Representatives Speaker Newt Gingrich, who saw a brief lead evaporate under a barrage of negative advertising last month, pledged to remain in the race and challenge Mr Romney, “a Massachusetts moderate”.
“We are not going to go out and run nasty ads,” said Mr Gingrich, who finished fourth.
“But I do reserve the right to tell the truth. And if the truth seems negative that may be more of a comment on his record than it is about politics.”
Mrs Bachmann, a Minnesota congresswoman who won the Iowa straw poll last summer, finished sixth, and urged voters not to let the media anoint a Republican nominee based solely on the Iowa results.
Former Utah Governor Jon Huntsman was on the ballot, but did not campaign in Iowa.
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