BISMARCK, N.D. — Donald J. Trump officially secured the number of delegates he needed to clinch the Republican presidential nomination Thursday, according to an Associated Press tally that showed unbound delegates putting him over the top.

Before a speech about energy in North Dakota on Thursday, Mr. Trump held a news conference intended to commemorate the symbolic elevation. He took questions from reporters for about 30 minutes, speaking with a backdrop of more than a dozen unpledged North Dakota delegates who are now supporting him.

“I’m so honored,” Mr. Trump said. “I’m so honored by these people. They had such great sense.”

Among the delegates behind Mr. Trump were John Trandem and State Representative Ben Koppelman, who were the final two to commit to Mr. Trump, helping to push him over the 1,237 delegate threshold. Each joked that he was the deciding delegate.

For Mr. Trump, whose remaining rivals dropped out the of the race this month, reaching the threshold puts an official capstone on a remarkable run in which he vanquished 16 other candidates, many of them with deep political experience. As recently as a month ago, the Republican race appeared to be heading for a contested convention, as a “Never Trump” effort sought to prevent him from winning enough support to get the nomination.