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.