Speaking at West Palm Beach, he told crowds: "Look what happened - is this crazy?" - adding: "I want to thank the American people for the extraordinary honour of being elected your 47th president.
"I will fight for you and your family and your future, every single day I will be fighting for you with every breath in my body."
He told the crowd it was a "magnificent victory for the American people" while claiming he had also won the popular vote, something he failed to secure during his 2016 and 2020 presidential bids.
"Winning the popular vote was very nice, it's a great feeling of love," he said.
The 78-year-old also told his supporters that "this will truly be the golden age of America".
Mr Trump has won in the key battleground states of Georgia, Pennsylvania and North Carolina - and is leading in several others - narrowing Kamala Harris's path to victory significantly.
According to Sky News's US partner network NBC, Mr Trump, who has been bidding for a second term in the White House following his win in 2016 against Hillary Clinton, is now just four electoral votes away from a projected win.
While the Republican Party's win of the Senate has been confirmed, the House is still up for grabs.
"We overcame obstacles that no one thought possible," Mr Trump said as family, including "my beautiful wife Melania" and his "amazing" children, stood next to him.
"We're going to help our country heal, we have a country that needs help and it needs help very badly," he said while doubling down on his promise to crack down on illegal immigration.
Mr Trump then seemed to refer to an attempted assassination on him at a rally in Pennsylvania back in July as he said: "Many people have told me that god has spared my life for a reason and that reason was to save our country and to restore America to greatness and now we are going to fulfill that mission together."
US election latest: Trump projected key wins
Ms Harris will now not address her supporters until later on Wednesday, her campaign chair has said.
The vice president had been scheduled to make a speech at her alma mater, Howard University, after the polls closed, but the mood grew sombre as results began to trickle in.
Cedric Richmond, co-chair of the Harris campaign, told the crowds at the university: "We will continue overnight to fight to make sure, every vote is counted, that every voice has spoken.
"So you won't hear from the vice president tonight but you will hear from her tomorrow. She will be back here tomorrow."
The Howard community had been preparing for a historic night - Ms Harris would become the first woman, black woman and South Asian American to assume the presidency, if elected.
But in the 21 remaining states where more than 80% of the vote has been counted, there has been a swing towards Mr Trump, NBC News reports.
In seven of them, the swing is less than one point.
Read more:
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America's night of historic firsts
What are the seven key battleground states?
The largest swings - of more than five points - are in New Jersey and Florida.
No other state has a swing higher than three points.
World reacts to looming Trump victory
World leaders have started reacting to Mr Trump's speech, with British Prime Minister Keir Starmer tweeting: "Congratulations president-elect Trump on your historic election victory.
"I look forward to working with you in the years ahead," he said.
"As the closest of allies, we stand shoulder-to-shoulder in defence of our shared values of freedom, democracy and enterprise.
"From growth and security to innovation and tech, I know that the UK-US special relationship will continue to prosper on both sides of the Atlantic for years to come."
Nigel Farage, leader of Reform UK and a longtime supporter of Mr Trump, tweeted: "He's done it again. The most incredible political comeback of our lifetime."
Israeli Prime Minister Benjamin Netanyahu, who will have been closely watching the election as he seeks continued support from the US over his war against the Hamas militant group in Gaza, said on X: "Congratulations on history's greatest comeback!"
Ukrainian President Volodymyr Zelenskyy, another leader who has been counting on renewed US support as he seeks to fend of Russia's full-scale invasion of his country, also congratulated Mr Trump over his looming victory, which he described as "impressive".
Mr Zelenskyy said on X he appreciated Mr Trump's "commitment to the 'peace through strength' approach" to global affairs and the principle could "bring just peace in Ukraine closer".
Indian Prime Minister Narendra Modi, Spain's Pedro Sanchez, European Commission President Ursula von der Leyen and Turkish President Recep Tayyip Erdogan were also among the leaders who congratulated Mr Trump on his claimed victory.
The UK prime minister said he looked forward to working with Mr Trump in the years ahead and said the relationship between the UK and US will "continue to prosper".
"As the closest of allies, we stand shoulder to shoulder in defence of our shared values of freedom, democracy and enterprise," he added.
US election latest: Trump declares victory in speech to supporters
Elsewhere, Ukraine's President Volodomyr Zelenskyy said he looked forward to "an era of a strong United States of America under President Trump's decisive leadership".
There had been questions about whether the US would continue supporting Ukraine, after former Russian president Dmitry Medvedev said a Trump win would probably be bad news for the country, which was invaded by Russia in 2022.
But Mr Zelenskyy said in a meeting with Mr Trump in September that the pair "discussed in detail the Ukraine-US strategic partnership, the victory plan, and ways to put an end to Russian aggression against Ukraine".
He has been joined in congratulating Mr Trump by Israel's prime minister Benjamin Netanyahu, France's president Emmanuel Macron and Hungary's prime minister Viktor Orban.
Asked during the election campaign how he would support Israel in its war against Hamas if re-elected, Mr Trump said: "Nobody's done what I've done for Israel."
Mr Netanyahu dubbed Mr Trump's win "history's greatest comeback", which offered "a new beginning for America and a powerful recommitment to the great alliance between Israel and America".
Hamas said Mr Trump's victory "puts him to the test" on his claims he could stop the war within hours.
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President Macron said he was "ready to work together" with Mr Trump, while Mr Orban called it a "much needed victory for the world".
Turkey's president Recep Tayyip Erdogan commended Mr Trump for winning "a great battle" and said he believed "more efforts will be given for a fairer world".
Meanwhile, Indian Prime Minister Narendra Modi congratulated his "friend" on his "historic election victory".
Read more:
The key moments of the US election night so far
Donald Trump vows to 'heal the country' in victory speech
During his first term in office, Mr Trump threatened to withdraw from the NATO alliance.
But its secretary general Mark Rutte said on Wednesday his leadership would be "key to keeping our alliance strong".
European Commission president Ursula von der Leyen "warmly" congratulated Mr Trump and said the European Union and the US were "more than just allies".
The result has not yet been confirmed, but his success in key battleground states, including Pennsylvania, means victory appears to be certain.
Follow live: Trump declares 'magnificent victory'
While Mr Trump headed to Palm Beach to address his supporters, aides for his Democratic presidential rival Kamala Harris said she would not be speaking tonight.
As the world waits for the result of the US election to be confirmed, these are the key moments of polling night so far.
Trump declares victory
Mr Trump, who was previously president between 2017 and 2021, gave a victory speech to a crowd of jubilant supporters in Florida - despite votes in key states still being counted.
However, at the time of the speech Trump had won the key battleground states of Georgia, Pennsylvania and North Carolina and was leading in several others.
The 78-year-old told a huge crowd of supporters that "this will truly be the golden age of America".
"We're going to help our country heal, we have a country that needs help and it needs help very badly," he said while doubling down on his promise to crack down on illegal immigration.
Ms Harris, the current vice president, had been scheduled to make a speech at Howard University but the mood among her supporters grew sombre as the night went on.
Cedric Richmond, co-chair of the Harris campaign, said she will now not address her supporters until later on Wednesday.
Trump takes first three swing states
The first key result came at around 11.30pm EST (4.30am UK) - a projected win for Mr Trump in North Carolina.
Republicans have won the battleground state for the last three elections - but always by a small margin.
An hour later Mr Trump secured another projected swing state victory in Georgia.
It's typically a Republican state, but President Joe Biden became the first Democrat to win it since Bill Clinton's win there in 1992.
Just after 2am local time (7am UK), Mr Trump took Pennsylvania, according to NBC projection data, the most valuable of the swing states in terms of Electoral College votes.
Republicans projected to win the Senate
Minutes after the Republicans' projected win in Georgia, it was announced they were also set to take control of the Senate.
NBC forecast that the GOP will take 51 seats to the Democrats' 40.
The fate of the House of Representatives is still undecided, but if Mr Trump wins the White House and both houses of Congress - he would be able to pass laws far more easily.
'Russian' bomb threats close polling stations
Polling stations in several of the seven battleground states were subject to bomb threats throughout Tuesday.
The security threats, which were all confirmed "non-credible" by the FBI, briefly stopped people from voting in Fulton County and DeKalb County, Georgia.
There were also hoaxes in Michigan, Wisconsin, and Arizona, but voting was not impacted there.
Two counties in North Carolina reported brief polling station closures due to computer and printer issues.
Both Georgia and North Carolina said they would have to extend voting hours as a result - but ultimately the official statewide polls closed on time - with some allowed to continue voting beyond the deadline.
Brad Raffensperger, Georgia's secretary of state, claimed the fake bomb threats had come from Russian IP addresses.
"They're up to mischief, it seems. They don't want us to have a smooth, fair and accurate election, and if they can get us to fight among ourselves, they can count that as a victory," he said.
A spokesperson for the Russian embassy in Washington described claims of interference as "malicious slander", adding: "As President Vladimir Putin has repeatedly stressed, we respect the will of the American people."
Russia has been accused of interfering in other recent US elections, particularly the 2016 race that saw Donald Trump beat Hillary Clinton.
Read more
What are the seven battleground states?
When will we know who's won?
The experts' views from key states
Internet claims a 'fake Melania' voted alongside Trump
When footage emerged on social media of Melania Trump voting alongside Mr Trump in Florida it was met with conspiracy theories it was not actually her.
Mr and Mrs Trump cast their ballots at Mandel Recreation Center in Palm Beach late on Tuesday afternoon.
But people online claimed it was a "fake Melania", questioning her appearance and speculating that Mr Trump had used a body double instead.
One post on X, which garnered thousands of views, captioned a clip of the former first lady: "That's not Melania. This is insane."
Sky News analysed the video and compared it with photos taken of Mrs Trump by the Reuters news agency - which show her by her husband's side at the same precinct - proving that it was her.
It's not the first time 'fake Melania' has trended online. Mrs Trump's rare appearances on this year's campaign trail - in stark contrast to the 2016 and 2020 races - sparked speculation the former president was using a lookalike to stand in for his wife at previous events.
Mrs Trump introduced her husband and X founder Elon Musk on stage at his rally at Madison Square Gardens late last month - and is reported to have taken part in private fundraisers, including one at Trump Tower in New York.
Florida fails to guarantee abortions - but other states succeed
A bid to guarantee abortion rights in Florida's constitution failed to get enough votes.
The proposal on the ballot would have allowed women to terminate pregnancies up to 24 weeks. Currently, the law only allows them up to six weeks - before most women know they are pregnant.
It's a victory for Republican Governor Ron DeSantis who has campaigned heavily against 'Amendment 4' with state-funded TV adverts and threats of criminal charges for TV channels that aired ads supporting it.
The amendment won majority support - but ultimately failed to get the 60% needed for it to become state law.
Abortion was on the ballot in 10 states after the Supreme Court voted to overturn Roe v Wade in 2022.
In contrast to Florida, Arizona has voted to change its abortion restrictions from 15 weeks to 24 weeks, according to NBC projections.
Democrat-safe states Colorado and New York were among those that voted to protect reproductive rights, NBC projected.
In New York, the constitution will now guarantee that people can't be denied their rights based on "pregnancy outcomes" as well as race, religion, and other protected characteristics.
In Colorado, reproductive rights will become part of the state constitution - barring state authorities from trying to impose a local ban - and the current constitutional ban against public-funded abortions will be lifted.
According to an NBC exit poll, only 14% of voters ranked abortion as the most important issue of the election overall - compared to 35% who said democracy.
Trump claims 'massive cheating' in Philadelphia
Donald Trump made more false claims of election fraud, saying there was "talk of massive cheating in Philadelphia" before polls closed there on Tuesday evening.
He said there was "law enforcement coming!" to investigate his claims in the Pennsylvania city - but there was no evidence of this.
Twenty minutes later he posted again on his Truth Social media platform that police were also out investigating electoral fraud claims in Detroit, Michigan.
The state's district attorney Larry Krasner said there was "no factual basis in these allegations".
"If Donald J. Trump has any facts to support his wild allegations, we want them now. Right now. We are not holding our breath," he wrote on X.
Both Pennsylvania and Michigan are among the seven battleground states that historically decide who wins the White House.
Mr Trump filed more than 60 lawsuits over claims of voter fraud in Pennsylvania, Michigan, Wisconsin, Arizona, Georgia, Nevada, and Washington DC in 2020.
It could be entitled 'Back to the Future'.
Donald Trump's base yearn for "the golden era", an America they believe has been lost.
He's on a mission to find it for them.
The New York tycoon stands on the precipice of power beyond his wildest imagination.
Young men, black men, Latino men have bought into Trump 2.0.
US election latest - Trump declares victory
The pollsters predicted a tight race. In the end, he's won by something resembling a landslide - with Republicans also flipping the Senate.
If they retain the House Of Representatives too, it will be an unchecked power - Mr Trump holding the keys to the White House and Congress.
Thousands of words could be written about Project 2025 - the conservative Heritage Foundation's plan for the radical reform of federal government.
But the pledge to 'Make America Great Again' again goes deeper and it is a campaign that has recruited the most unlikely of poster boys.
Elon Musk - the billionaire tech mogul and the world's richest man - is on board and could soon take charge of the Department of Government Efficiency.
Robert F Kennedy Jr, who is estranged from the rest of his political dynasty, suspended his own run for the White House to back Mr Trump.
"RFK" is renowned for questioning the safety of drinking water and vaccines.
The president-elect has not ruled out giving him responsibility for health.
In election campaign speeches, Mr Trump has laid out his plans to invest in flying cars and build "freedom cities" - free from government regulations.
On immigration, he promised the biggest mass deportation of illegal migrants in US history, to finish building the wall and enforce strict border control.
Mr Trump has vowed to relocate rough sleepers from city centres to tented camps until their "problems can be identified".
But it's the culture war mongering that will cause the most alarm - no constitutional right to abortion; teachers required to "embrace patriotic values".
Mr Trump has an isolationist policy and seeks to detangle America from global conflicts, not least in Ukraine and the Middle East.
Read more:
Republicans gain control of Senate
America's night of historic firsts
He has pledged to cut "hundreds of billions" of dollars in international aid, end the war in Ukraine, and potentially withdraw America from NATO.
He had already made history - the first president to become a convicted felon. Only one other has won two non-consecutive terms.
The unconventional, unfiltered 45th president of the United States will be the 47th president and the rest of the world holds its breath.
A third of the Senate's 100 seats were up for grabs at this year's elections, with 51 required for a majority.
Senate Republicans ousted Democrats in red states to secure the majority, flipping seats in West Virginia and Ohio.
They also held their ground in friendly states like Texas and Florida, assuring them at least 51 seats when the new Congress is sworn in next January.
Democrats, who have been in control of the Upper House of Congress for the last four years, entered election day with a 51-49 edge
However, with eight seats yet to be declared, the Republicans have 51 seats in the Senate and the Democrats have 40.
The Senate, similar to the House of Lords in the UK but is elected, is made up of two senators for each state.
US election latest: Trump addressing 'jubilant' supporters
Independent candidate Bernie Sanders, who sits with the Democratic caucus, has been projected to win a fourth term representing Vermont.
Among the Republicans' gains were Trump-backed Bernie Moreno, who flipped a seat in Ohio which had been held by the Democrats since 2007.
Another gain was in West Virginia where Jim Justice won a seat that was left vacant after independent senator and former Democrat Joe Manchin's retirement.
All 435 seats in the House of Representatives, the lower house, are also up for grabs.
Read more:
Trump declares victory in presidential election
The key moments of the US election night so far
The Republicans' gains in the Senate come as Donald Trump has been projected to win several key battleground states in the race to the White House, closing in on a second presidential term.
He declared victory in a speech to supporters in Florida, while Kamala Harris will not speak until later on Wednesday.