Former FBI chief James Comey had been investigating possible collusion between Mr Trump’s election campaign officials and Moscow.
The President insists Mr Comey told him repeatedly that he was in the clear before he sacked him on Tuesday.
Mr Trump’s denial came as the turmoil surrounding Mr Comey’s dismissal showed no sign of abating.
Mr Trump told NBC news he had asked Mr Comey whether he was an FBI target, twice over dinner and once over the telephone.
“I said: ‘If it’s possible, would you let me know: am I under investigation?’
“He said: ‘You are not under investigation’.
“We had a very nice dinner. And at that time, he told me, ‘You are not under investigation’.”
Mr Trump added: “He’s a showboat. He’s a grandstander. The FBI has been in turmoil. You know that, I know that, everybody knows that.”
He added: “It’s an excuse by the Democrats for having lost an election that they should have won.”
He said there was no “collusion between me and my campaign and the Russians” and “the Russians did not affect the vote”.
FBI officials are looking into whether the Kremlin interfered in last year’s election to help Republican Mr Trump beat Democrat Hillary Clinton.
In the run-up to the vote, Mr Trump was accused of having close links to the Putin regime for as long as five years.
Today acting FBI chief Andrew McCabe told the Senate Intelligence committee Mr Comey had “broad support” from the rank and file.
Mr McCabe promised to expose any White House meddling into the probe on Russia.
He added: “It is my belief that the FBI will continue to pursue this investigation vigorously and completely.”