The New York billionaire reached the number of delegates needed to clinch the GOP nomination hours earlier, according to the Associated Press count, just before a North Dakota campaign stop.
It completed his unlikely rise that has upended the political landscape and set the stage for a bitter fall campaign.
"Here I am watching Hillary fight, and she can't close the deal," he told reporters. "We've had tremendous support from almost everybody."
Trump was put over the top in the AP delegate count by a small number of the party's unbound delegates who told the AP they would support him at the national convention in July. Among them was Oklahoma GOP chairwoman Pam Pollard.
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"I think he has touched a part of our electorate that doesn't like where our country is," Pollard said. "I have no problem supporting Mr Trump."
Trump, a political neophyte who for years delivered caustic commentary on the state of the nation from the sidelines but had never run for office, fought off 16 other Republican contenders in an often ugly primary race.
Many on the right have been slow to warm to Trump, wary of his conservative bona fides. Others worry about his crass personality and the lewd comments he's made about women.
Steve House, chairman of the Colorado Republican Party and an unbound delegate who confirmed his support of Trump to the AP, said he likes the billionaire's background as a businessman.
"Leadership is leadership," House said. "If he can surround himself with the political talent, I think he will be fine.