"President Barack Obama was born in the United States, period," Trump said, as his campaign tried to silence damaging charges of racism.
Trump pointedly did not apologise, and repeated false allegations that Hillary Clinton started the so-called "birther" movement questioning Obama's nationality - and therefore legitimacy as president.
"Her campaign of 2008 started the birther controversy. I finished it. I finished it. You know what I mean," Trump said.
Trump, Clinton neck and neck in new poll
Since at least 2011, the celebrity businessman has questioned Obama's US citizenship, which is a legal prerequisite for becoming president.
The unfounded claims were a launchpad for Trump's political career, propelling him onto the national stage and winning him fans on the far right.
But in a tight election race, Trump's position has become a liability, repulsing black, Hispanic and moderate voters who he needs to win the Oval Office.
After Trump on Thursday again demurred on Obama's nationality, his campaign was forced to issue a statement.
"Mr. Trump believes that President Obama was born in the United States," his campaign said.
Trump, Clinton neck and neck in new poll
Trump's Democratic rival Hillary Clinton jumped on that as more evidence Trump is unfit to be president.
"For five years, he has led the birther movement to delegitimize our first black president," she said. "His campaign was founded on this outrageous lie. There is no erasing it in history.
"He's feeding into the worst impulses, the bigotry and bias that lurks in our country," she added. "Barack Obama was born in America, plain and simple. And Donald Trump owes him and the American people an apology."
Trump's brief remarks on the subject came at the end of a lengthy campaign event that plugged his new hotel in the capital Washington, and his support from veterans, but scarcely touched on the "birther" issue.
The White House has long viewed Trump's claims as racist, aimed at delegitimising the president.
Asked in the Oval Office about the renewed controversy Friday, Obama gave the question short shrift and made a veiled dig at the US media and its obsessive coverage of Trump.
"I am shocked that a question like that would come up at a time when we have so many other things to do," he said, before adding "Well I am not that shocked."
"I was pretty confident about where I was born. My hope would be that the presidential election reflects more serious issues than that."
In 2011, Obama produced his birth certificate - showing he was born at the Kapiolani Medical Center in Hawaii on August 4, 1961 -- to put an end to the allegations.
Shortly after, Obama appeared at the White House Correspondents' Dinner and publicly ridiculed Trump, who was in the audience.
"No one is prouder to put this birth certificate matter to rest than The Donald. And that's because he can finally get back to focusing on the issues that matter, like, did we fake the moon landing? What really happened in Roswell? And where are Biggie and Tupac?"
'Trump's actions disgraceful'
US Democratic presidential nominee Hillary Clinton's campaign on Friday blasted Republican rival Donald Trump's statement acknowledging that President Barack Obama was born in the United States after years of questioning the president's citizenship.
"Trump's actions today were disgraceful," Clinton campaign manager Robby Mook said. "After five years of pushing a racist conspiracy theory into the mainstream, it was appalling to watch Trump appoint himself the judge of whether the President of the United States is American."
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