A campaign spokeswoman said Corey Lewandowski would no longer be working on it and said the team was grateful to him "for his hard work and dedication".
Speaking on CNN, he did not deny being fired, replying "I don't know" when asked why why he was let go.
He oversaw the New Yorker's unlikely triumph in the primary contests.
The exact reason for Mr Lewandowski's departure is not yet clear but he denied it was to do with his abrasive style.
His departure comes as the businessman faces strong resistance from senior members of his own party over his strident tone, hardline immigration policy and falling poll numbers.
Mr Lewandowski was not escorted from the building as some reported but accompanied by a "friend from the campaign", he told CNN, saying it had been a privilege to work on the team.
He also denied he had not been getting along with top Trump strategist Paul Manafort and the Trump children.
Mr Lewandowski has had a contentious relationship with the press. Earlier this year, he was charged with battery after allegedly yanking a female reporter out of Mr Trump's way after a campaign event. The charges were dropped.
One Trump campaign staff member told NBC News that the campaign was not briefed on Mr Lewandowski's firing and that right now there is "bedlam in the Trump campaign. No one knows what is happening".
Republicans have started to distance themselves from Mr Trump following his personal attacks against a Hispanic federal judge overseeing two lawsuits against him.
Among his critics is Speaker of the House and top-ranked Republican Paul Ryan, who appeared to leave the door open to a possible revolt at next month's convention.
When asked on Sunday about reports that party delegates may rebel against voting for Mr Trump at the convention, he said: "They write the rules, they make the decisions."
Mr Trump is suffering in political polls lately, with most voters viewing him as "strongly unfavourable".
He is likely to face Democrat Hillary Clinton in November's election, and there were reports over the weekend that her campaign is way ahead in spending in the key swing states.