The billionaire businessman had been expected to cruise past the 1,237 delegates needed on Tuesday to seal the deal on the first ballot.
Trump was put over the top by his home state of New York.
“It is something I’ll never, ever forget,” Trump said on a video feed from New York.
“Together we have achieved historic results with the largest vote total in the history of the Republican Party. This is a movement, but we have to go all the way.”
Anti-Trump forces on the floor held out for a final miracle on Tuesday after seeking to convince delegates that their votes were not bound and that they could vote with their conscience, but it never came to fruition.