The former ambassador to the United Nations and South Carolina governor trumped the former president by just one vote in Johnson County, with 1,271 votes to 1,270 and 99 per cent of votes tallied.
Florida Governor Ron DeSantis trailed behind both candidates in the county with 755 votes.
Overall, the former president enjoyed a monumental win in the first-of-the-nation caucus, with 51 per cent of votes to Mr DeSantis 21.2 per cent and Ms Haley’s 19.1 per cent, the Associated Press projected as of 5am ET.
It marks a historic, unprecedented margin of the votes for the former president and firmly cements his position as the Republican frontrunner.
It took less than an hour for him to be projected the winner – coming at a time when he is facing 91 charges across four criminal cases.
In his victory speech, the former president couldn’t resist subtly gloating about his monumental win, as he congratulated his GOP rivals for “having a good time together” at the back of the race.
“I want to congratulate Ron and Nikki for having a good time together… We don’t even know what the outcome of second place is,” he said.
Mr Ramaswamy swiftly announced he was dropping out of the Republican primary race off the back of his dismal performance, and threw his endorsement behind Mr Trump.
“This entire campaign is about speaking the TRUTH. We did not achieve our goal tonight & we need an America-First patriot in the White House. The people spoke loud & clear about who they want,” he wrote on social media.
“Tonight I am suspending my campaign and endorsing Donald J. Trump and will do everything I can to make sure he is the next U.S. President. I am enormously proud of this team, this movement, and our country.”
Following his endorsement, Mr Trump – who has launched a series of attacks against Mr Ramaswamy in recent days – thanked him, posting on Truth Social: “Thank you Vivek, a Great Honor!”
Mr Trump’s Maga allies are now calling on Mr DeSantis and Ms Haley to follow suit, claiming that they are using up valuable donor money in an ultimately futile race.
Florida Rep Matt Gaetz and Georgia Rep Marjorie Taylor Greene joined Mr Trump at his watch party in Des Moines on Monday night, with the former telling CNN that the Republican primary is now “over” for the other candidates.
“It’s been real. You know, tip your waitresses on the way out, but this primary is over” he said.
“This race is over. Donald Trump has consolidated the Republican Party. Ron DeSantis was getting his mail forwarded here. Like he had moved here. What is his argument to continue this campaign?”
Ms Greene also told the network that Ms Haley and Mr DeSantis should drop out.
“I think donor money is really important. And political consultants are pretty good at talking candidates into staying in races way longer than they actually should,” she said.
But both Ms Haley and Mr DeSantis have expressed their refusal to drop out, with the former UN ambassador saying that her sights are set on New Hampshire’s primary next week.
While they set off for the state in a hope to claw back their campaigns – and offer an alternative to Mr Trump – the former president is headed for New York to face the start of his defamation trial against magazine columnist E Jean Carroll.