Trump has fired James Comey, the FBI director, over his handling of the investigation into Hillary Clinton’s emails, a dramatic dismissal of the man in charge of investigating the Trump campaign's connections with Russia.
The US president said in a letter to Mr Comey that he was "not able to effectively lead the bureau".
The Trump campaign is currently being investigated by the FBI over allegations that it colluded with Russia to influence the outcome of the election.
"While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the bureau," Mr Trump said.
"It is essential that we find new leadership for the FBI that restores public trust and confidence in its vital law enforcement mission."
Mr Comey was speaking to agents at the FBI's field office in Los Angeles when the news of his firing flashed on TV screens, according to a law enforcement official who was there.
Laughing, the FBI director was reported as saying he thought it was a prank and made a joke about it to lighten the mood, sources told CNN and the Times.