Mr Trump suggested Mr Obama's US birth cert may have been "a fraud"
Hillary Clinton has fired back, after Donald Trump finally accepted that Barack Obama was born in the US.
The Republican presidential candidate had claimed for years that Mr Obama was born outside the country, and therefore was ineligible to be president.
Mr Trump tweeted in 2012 that the birth certificate may have been "a fraud" and continued to raise questions about its veracity.
An 'extremely credible source' has called my office and told me that @BarackObama's birth certificate is a fraud.— Donald J. Trump (@realDonaldTrump) August 6, 2012
But today, Mr Trump backtracked, telling a rally in Washington DC: "President Barack Obama was born in the United States. Period.
"Now we all want to get back to making America strong and great again."
Donald Trump: "President Barack Obama was born in the United States. Period." https://t.co/d6Gd2Y8UL8— MSNBC (@MSNBC) September 16, 2016
But Mr Trump's Deomcratic rival Hillary Clinton hit back, saying: "What Trump just did is a disgrace.
"Trump has spent years peddling a racist conspiracy aimed at undermining the first African-American president. He can't just take it back.
"The birther lie is what turned Trump from an ordinary reality TV star into a political figure. That origin story can't be unwritten.
"Trump’s birtherism stems from the same innate beliefs that led to discriminating against black tenants early in his career. (It) can't be undone.
"What Trump should do: for once in his life, own up to his mistakes. Apologise to the President, and to the American people."
Expressing zero regret for years of pushing a racist conspiracy theory, Trump again appointed himself judge & jury on @POTUS's citizenship.— Hillary Clinton (@HillaryClinton) September 16, 2016
It comes as Mr Trump Mrs Clinton released statements on their health, after Mrs Clinton was diagnosed with pneumonia.
Harold N Bornstein, Mr Trump's doctor, said the tycoon has been in hospital only once, when he had an appendectomy at the age of 11.
He said Mr Trump weighs 236 pounds and is six feet three inches tall - which the candidate earlier admitted made him overweight for his height.
While Mrs Clinton's doctor, Lisa Bardack, said she is "healthy and fit to serve as president".
Dr Bardack said the Democratic nominee was recovering well from "mild, non-contagious bacterial pneumonia" and had been prescribed a 10-day course of antibiotics.
While the US Secretary of State John Kerry said he is "absolutely convinced" Mrs Clinton is up to being president after her health scare.