The US billionaire developer and reality show celebrity ruled out the possibility that he will run a third-party campaign if losing the party's nomination, Xinhua news agency reported.
"I will be totally pledging all my allegiance to the Republican Party and the conservative principles to which it stands," Trump said at a press conference in New York.
"We will go out and we will fight hard and we will win, and most importantly, we will make our country great again," Trump said to his cheering supporters.
Trump's pledge of not running a third-party bid ended a month-long drama climaxing with his public defiance of such a pledge on August 6 in front of about 24 million TV viewers during the first Republican presidential primary debate.
For the Republican National Committee (RNC), which sent out written pledges to Republican candidates on Wednesday, Trump's pledge matters more than others.
Standing atop almost every national poll for months, with a double-digit lead over the crowded Republican field, Trump's popularity shows no trace of ebbing away.