The decision leaves Mr Trump and former UN ambassador Nikki Haley as the last major candidates remaining in the race ahead of Tuesday’s New Hampshire primary.
This is the scenario Mr 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.
But as some Trump critics cheered, Mr DeSantis nodded towards Mr Trump’s primary dominance – and attacked Ms 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,” Mr DeSantis said in the straight-to-camera video, delivered in a cheerful tone, through forced smiles.
He continued: “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.”
Ms Haley spoke at a campaigning stop in Seabrook, New Hampshire, just as Mr DeSantis announced his decision.
“He ran a great race, he’s been a good governor, and we wish him well,” she told a room packed with supporters and media.
“Having said that, it’s now one fella and one lady left.“
Mr DeSantis’s decision, while perhaps 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 Mr Trump’s supremacy in the Republican Party.
He entered the 2024 presidential contest with major advantages in his quest to take on Mr Trump, and early primary polls suggested Mr DeSantis was in a strong position to do just that.
He and his allies amassed a political fortune well in excess of 130 million dollars (£102 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, Mr 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 Mr Trump.
Mr DeSantis’s 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 Mr 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 Mr DeSantis, who is in his second and final term as Florida’s governor, which ends in January 2027.
The endorsement was a stunning tail-between-his-legs moment for Mr DeSantis, whom Mr Trump has mercilessly and relentlessly taunted in deeply personal terms for the better part of a year now.
For Mr Trump, whose team includes many former DeSantis staff members, the attacks have often felt more like sport than political strategy.
Mr 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.
Mr DeSantis’s team joined Mr Trump in attacking Ms Haley as news of his departure rippled across the political landscape.
Some doubt Ms Haley, who was seen as splitting Republican votes and preventing a head-to-head match up with Mr Trump, would benefit from Mr DeSantis’s decision.
“She will not be the nominee,” key DeSantis supporter Representative Chip Roy (Republican-Texas) told The Associated Press.
“She will not be the president of the United States.”
Mr Trump had already shifted his focus to Ms Haley in recent weeks, but minutes after Mr DeSantis’s announcement, the former president’s campaign released a new memo highlighting the pressure on Ms Haley to win New Hampshire.
“Now that we are a mere 48 hours from the primary, the tone has shifted mightily. We see it, you see it, but make no mistake, if Nikki Haley loses in New Hampshire – there are only two options,” wrote senior advisers Chris LaCivita and Susie Wiles.
“Option A: Nikki Haley drops out, unites behind President Trump, and commits to defeating Joe Biden,” they wrote.
“Option B: Nikki Haley prepares to be absolutely DEMOLISHED and EMBARASSED in her home state of South Carolina,” which votes on February 24.
“Now, for some important advice,” they continued.
Mr Trump himself addressed Mr DeSantis’s 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,” Mr 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”, Mr Trump said to loud cheers: “I said that name is officially retired.”