The billionaire is about to go on stage to give a victory speech.
Clinton supporters were seen stony-faced or crying at what was supposed to be her victory rally in New York before being sent home by her campaign manager.
John Podesta insisted her campaign was “not done yet” and would have more to say once Americans wake up.
“They are still counting votes, and every vote counts,” he told the crowd.
But across town a triumphant Mr Trump is preparing to address his supporters.
Early exit polls had provided the Democrats with optimism, but shortly after 2am UK time the momentum began to shift as it emerged Mr Trump was edging ahead in a number of key states.
The first swing state to be called for Mr Trump was Ohio at just before 3.30am – and Florida, Iowa, North Carolina and Pennsylvania all followed for the billionaire.
Mrs Clinton looks set to win the popular vote across the nation – but it is Electoral College votes that count.
World markets began to respond as the US map turned Republican red, with falls reported across Asia and the Mexican peso dropping to a record low against the dollar.
Mr Trump will not be inaugurated until 20 January – but thoughts are already turning to what his presidency will look like following promises to build a wall at the Mexican border and to ban Muslims from entering the US.
The 70-year-old will be the oldest person ever inaugurated as US president.
On what turned out to be a dream night for the Republicans, the party also retained control of the Senate and the House of Representatives.