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Donald Trump signs the pledge.
The pledge, circulated by the Republican National Committee to GOP candidates, is aimed at boxing in Trump. In the pledge, candidates affirm that they will not run as third-party candidates if they don't win the Republican nomination.
Trump has multiple times threatened to run as a third-party candidate in the event Republicans do not treat him "fairly" and attempt to push him out of the race.
But Trump said Thursday that he was satisfied with how he had been treated by the national party thus far.
"The RNC has treated me with great respect," Trump said.
"I felt that the best way to win and to beat the Democrats," he added, was to sign the pledge.
Trump's campaign subsequently sent out a statement saying that it was a "great honor" to adhere to the pledge.
"It is my great honor to pledge my total support and loyalty to the Republican Party and the conservative principles for which it stands," he said. "This is far and away the best way to secure victory against the Democrats in November 2016. I am leading in all local and national polls - my whole life has been about winning and this is what must be done in order to win the election and, most importantly, to make America great again!"
The relationship between Trump and the RNC has been fraught since his bombastic entry into the race in mid-June. He has drawn intense scrutiny over his comments on immigration, his remark on the war record of US Sen. John McCain (R-Arizona), and his feud with Fox News host Megyn Kelly.
REUTERS/Lucas Jackson
US presidential hopeful Donald Trump holds a signed pledge during a press conference at Trump Tower in Manhattan Thursday. He will not run as an independent candidate if he loses the Republican Party nomination.
Trump has said all along that he wants to run as a Republican, but he has floated the third-party threat as "leverage."
The first Republican presidential debate last month began with fireworks when the candidates were asked by Fox News moderator Bret Baier whether they would vow to support the eventual GOP nominee and "pledge to not run an independent campaign against that person."
Only Trump raised his hand to indicate he couldn't make that commitment.
"I cannot say I have to respect the person that - if it's not me - the person that wins," Trump said.
"If I do win, and I'm leading by quite a bit, that's what I want to do. I can totally make that pledge: If I'm the nominee, I will pledge that I will not run as an independent," Trump said, adding, "and I am discussing it with everybody. I'm talking about a lot of leverage. We want to win and we will win. But I want to win as the Republican. I want to run as the Republican nominee."
As some observers have noted, the pledge is not a binding contract but only a political document. Trump did say, however, that he sees "no circumstances under which I would tear up that pledge."