Manafort is a longtime Republican adviser
US presidential candidate Donald Trump has confirmed his campaign chairman Paul Manafort has resigned.
Mr Trump says Mr Manafort offered his resignation this morning, which he accepted.
"I am very appreciative for his great work in helping to get us where we are today, and in particular his work guiding us through the delegate and convention process," Mr Trump said in a statement.
"Paul is a true professional and I wish him the greatest success," he added.
The 67-year-old joined the Trump campaign in March. The reasons behind his departure are not yet known.
But it comes amid another shake-up of the campaign, the second in less than two months.
The head of conservative news website Breitbart News, Steve Bannon, was this week appointed campaign chief executive officer - seen by some as pushing Manafort aside.
While senior adviser Kellyanne Conway was promoted to the post of campaign manager.
Mr Manafort is a longtime Republican adviser, whose work with a pro-Russian party in Ukraine has come under increasing scrutiny.
Meanwhile, in another change of tactics, the businessman has expressed 'regret' for sometimes saying the 'wrong thing' during his campaign.
"Sometimes in the heat of debate and speaking on a multitude of issues, you don't choose the right words or you say the wrong thing.
"I have done that. And, believe it or not, I regret it, and I do regret it. Particularly where it may have caused personal pain," he told an audience in North Carolina.
It comes as Hillary Clinton holds a 6-percentage-point lead over her Republican rival in the latest Reuters/Ipsos poll.
However this - and several other poll results - have been disputed by special counsel at the Trump Organisation, Michael Cohen.