The decision leaves Trump and former UN ambassador Nikki Haley as the last major candidates remaining in the race before Tuesday's New Hampshire primary.
This is the scenario Trump's foes in the Republican Party have long sought, raising the stakes for this week's contest as the party's last chance to stop the former president who has so far dominated the race.
"Success is not final, failure is not fatal: it is the courage to continue that counts."— Ron DeSantis (@RonDeSantis) - Winston Churchill pic.twitter.com/ECoR8YeiMmJanuary 21, 2024
But as some Trump critics cheered, DeSantis nodded towards Trump's primary dominance - and attacked Haley - in an exit video he posted on social media.
"It's clear to me that a majority of Republican primary voters want to give Donald Trump another chance," DeSantis said in the video, delivered in a cheerful tone through forced smiles.
"I signed a pledge to support the Republican nominee and I will honour that pledge.
"He has my endorsement because we can't go back to the old Republican guard of yesteryear, a repackaged form of warmed-over corporatism that Nikki Haley represents."
Haley spoke at a campaigning stop in Seabrook, New Hampshire, on Sunday just as DeSantis announced his decision.
"He ran a great race, he's been a good governor, and we wish him well," she said.
"Having said that, it's now one fella and one lady left."
DeSantis' decision, while not surprising given his 30-point blowout loss last week in Iowa, marks the end of an extraordinary decline for a high-profile governor once thought to be a legitimate threat to Trump's supremacy in the Republican Party.
Donald Trump has relentlessly taunted DeSantis for the better part of a year. (AP PHOTO)
He entered the 2024 presidential contest with major advantages in his quest to take on Trump, and early primary polls suggested DeSantis was in a strong position to do just that.
He and his allies amassed a political fortune well in excess of $US130 million ($A152 million), and he boasted a significant legislative record on issues important to many conservatives such as abortion and the teaching of race and gender issues in schools.
Such advantages did not survive the reality of presidential politics in 2024.
From a high-profile announcement that was plagued by technical glitches to constant upheavals to his staff and campaign strategy, DeSantis struggled to find his footing in the primary.
He lost the Iowa caucuses - which he had vowed to win - by 30 percentage points to Trump.
DeSantis' allies said that private discussions began shortly after Iowa to decide how to bow out of the race gracefully.
They ultimately decided that he needed to endorse Trump despite the deeply personal feud between them.
"While I've had disagreements with Donald Trump, such as on the coronavirus pandemic and his elevation of Anthony Fauci, Trump is superior to the current incumbent, Joe Biden. That is clear," said DeSantis, who is in his second and final term as Florida's governor, which ends in January 2027.
Nikki Haley is the only major candidate left to fight Trump for the Republican nomination. (AP PHOTO)
The endorsement was a stunning tail-between-his-legs moment for DeSantis, whom Trump has relentlessly taunted in deeply personal terms for the better part of a year.
Trump and his aides have blasted the governor as disloyal for running in the first place, mocked his eating habits and his personality and accused him of wearing high heels to boost his height.
Trump addressed DeSantis' departure shortly after it was announced during a stop at Manchester campaign headquarters, according to a video shared by his staff.
"We just got some word that one of our opponents, very capable person, is dropping out of the race - Ron DeSantis. And Ron is dropping out and, in doing so, he endorsed us," Trump said to cheers, before a "Trump!" chant broke out in the room of volunteers.
Asked if he would be using the name "Ron DeSanctimonious", Trump said to loud cheers: "I said that name is officially retired."
