Reports say this was on the recommendation of the US Attorney-General
Donald Trump has fired FBI boss James Comey, with a White House statement saying he had been "terminated and removed from office".
He was dismissed on the recommendation of US Attorney-General Jeff Sessions and the Deputy Attorney-General, the statement added.
In a letter to Mr Comey, the president told him he was "not able to effectively lead" the FBI and that new leadership was needed to restore trust.
"The FBI is one of our Nation's most cherished and respected institutions and today will mark a new beginning for our crown jewel of law enforcement," said President Trump.
The search for Mr Comey's successor will start immediately.
Mr Comey (56) was nominated by Mr Trump's predecessor Barack Obama in 2013 for a 10-year term, having served three decades in law enforcement.
Mr Trump's Democratic rival for the presidency, Hillary Clinton, accused Mr Comey of ruining her chances of winning by revealing a renewed investigation into her e-mail use at a crucial time in the campaign.
The decision to sack Mr Comey will raise questions about Mr Trump's motives, as he had been leading the FBI in investigating the Trump campaign's possible links to Russia.