Huntsman made the announcement in South Carolina, site of the critical first Southern primary this Saturday. His decision could provide an extra cushion of votes for Romney, who is facing stiff conservative competition from Rick Santorum and Newt Gingrich.
Romney's campaign already is bolstered by a huge campaign war chest and a wave of momentum after wins in Iowa and New Hampshire. It's hoping for a possible knockout blow with a first place finish in the Palmetto State. South Carolina has picked the winner of every GOP presidential nomination fight since 1980.
"Governor Huntsman did not want to stand in the way of the candidate best prepared to beat Barack Obama and turn our economy around. That's Mitt Romney," a senior Huntsman campaign official told CNN.
The endorsement represents a notable shift for Huntsman. He recently declared Romney "completely unelectable" during an interview on CNBC.
"Jon Huntsman made a basic calculation here, let's be honest," said CNN Chief National Correspondent John King. "If you look at the polling, he's doing miserable in South Carolina. He's not going to win the state next Saturday and so if he wants to run in 2016 ... this is a chance ... to try to get a little bit of goodwill in the party, a little bit of leverage."
An American Research Group poll released Friday showed Huntsman with 1% of likely GOP primary voters in South Carolina.
The impact of a Huntsman withdrawal after South Carolina also could have been minimized and overshadowed by the possible withdrawals of more conservative candidates, including Santorum, Gingrich, or Rick Perry.
Huntsman -- a former ambassador to China -- surprised a number of political observers when he initially decided to compete in South Carolina following his distant third-place finish in the New Hampshire primary. While Huntsman characterized his New Hampshire finish as a victory, most pundits saw it as a disappointment. The former governor had devoted virtually all of his time and resources to the Granite State.
Huntsman's path forward after New Hampshire was unclear. He had little money and a minimal campaign infrastructure in South Carolina, and showed no sign of making inroads with the GOP's conservative base. He also had trouble developing a consistent message after launching his presidential campaign last June.
Despite his low polling numbers, South Carolina's largest newspaper, The State, endorsed Huntsman on Sunday.
The paper's editorial board stated that while Romney is "more appealing" than the rest of the GOP field, Huntsman is "more principled, has a far more impressive résumé and offers a significantly more important message."
The board praised what it called the "essential values that drive his candidacy: honor and old-fashioned decency and pragmatism."
R.C. Hammond, a spokesman for Gingrich, said that with Huntsman dropping out, "we are one step closer to a bold Reagan conservative winning the GOP nomination."
For their part, Gingrich and the other remaining conservative hopefuls stepped up their attacks on Romney on Sunday, to try to head off a third straight win for the former Massachusetts governor.
In appearances on morning talk shows, Gingrich, Santorum and Perry all contrasted their conservative credentials with what they characterized as Romney's more moderate gubernatorial record.
Debates on Monday in Myrtle Beach and Thursday in Charleston will likely give voters their last chance to see all the remaining Republican candidates on one stage, as some are considered likely to drop out after Saturday's vote. CNN will broadcast the Charleston debate.
Two South Carolina Republican politicians, Sen. Lindsey Graham and Rep. Tim Scott, said Sunday that another Romney victory this week would likely sew up the nomination for him.
"If for some reason he's not derailed here and Mitt Romney wins South Carolina, no one's ever won all three, I think it should be over," Graham told NBC's "Meet the Press" program. "That would be quite a testament to his ability as a candidate and a campaigner."
On the same program, Scott said: "If Romney wins South Carolina, I think the game is over."
Perry, who has lagged in the polls since a series of poor debate performances last fall, kept up his criticism Sunday of Romney's experience as a venture capitalist -- an attack line shared by Gingrich but criticized by Santorum and other conservatives.
After previously describing Romney's former company, Bain Capital, as corporate "vultures," Perry used less harsh language during an appearance on CNN's "State of the Union." He reiterated his accusation, however, that Bain "came in and basically shut down" a South Carolina steel company, taking away "a lot of money in management fees."
Obama's campaign will raise the matter if Romney gets the Republican nomination, Perry warned.
"The issue's not going away," he said. "Now's the time to talk about it, not in September and October."
Gingrich, a former U.S. House speaker, expressed a similar view on NBC's "Meet the Press" and CBS's "Face the Nation."
"Our nominee had better be capable of standing up there, telling the truth, enduring the negative ads and winning the vote," Gingrich said, insisting he is a more accomplished debater and stronger candidate than Romney.
Gingrich also told NBC he would release his tax returns on Thursday, and reiterated his challenge for Romney to do the same -- continuing a campaign to get Romney to disclose details of his substantial personal wealth.
Romney has said he has complied with legal disclosure obligations, but added he might release his tax returns in the future.
Gingrich said it would be better for Romney to release his tax returns now instead of having the issue come to a head in the fall during a one-on-one campaign against Obama.
Santorum, meanwhile, told "Fox News Sunday" that an endorsement on Saturday from Christian conservative leaders should help his campaign, as he seeks to regain the luster of a razor-thin second-place finish behind Romney in Iowa.
Santorum, who finished fifth in New Hampshire, has been fighting with Gingrich and Perry for support from South Carolina's powerful evangelical voters. Saturday's endorsement was intended to unite those voters behind one candidate and avoid a split that would hand Romney a victory despite South Carolina's conservative pedigree.
"It would be helpful if everybody dropped out and I would win," Santorum said. "But, you know, the idea is, we're going to go through this process, people have the right to go out and make the case to the voters and then we'll see what happens."
The American Research Group poll released last week finds Romney and Gingrich in a statistical dead heat in the state.
According to the poll, 29% of likely GOP primary voters say they will support Romney. Another 25% said they would support Gingrich, putting Romney's lead within the poll's sampling error.
The survey indicates Rep. Ron Paul of Texas has 20% of the vote, Perry has 9%, Santorum has 7%, and 7% are undecided.