According to BBC, in North Dakota on Thursday, he thanked 15 unbound delegates from the state who he said "got us right over the top".
He defeated 16 other Republican contenders and according to the Associated Press has 1,238 delegates, one more than needed.
Republicans will finalise their nomination at a convention in July.
However while Trump has the required amount of delegates, his nomination by a divided Republican Party is not yet secured.
Unbound delegates in the party are free to support the candidate of their choice. BBC reports.
If his nomination is confirmed, Trump will face former US Secretary of State Hillary Clinton or Vermont Senator Bernie Sanders, who are vying for the Democrat nomination.
According to BBC, on Wednesday, the New York billionaire suggested going against Mr Sanders in a TV debate in California before the state's primary on 7 June.
Mr Sanders agreed to the debate in a tweet, saying "Game on". Trump also did tweet saying "the problem with Sanders is that he will lose..."