" I was going to fire Comey - my decision", Mr Trump told NBC.
He also said Comey is "a showboat".
Comey has not confirmed Trump's account.
Trump's tweet about Comey was likely a reaction to a New York Times report, published Thursday night, detailing a one-on-one dinner Trump had with Comey shortly after the inauguration. Comey declined, instead offering "honest".
Yesterday however, Mr Trump said he would have taken the action regardless.
Trump has revealed he asked Comey on three occasions whether he was a target in a probe into alleged Russian election meddling, stoking allegations of presidential interference. The president also did not indicate who he believes may have recorded the two men's conversation. Trump's firing of Comey already has left him with the dubious distinction of being the first president since Nixon to fire a law enforcement official overseeing an investigation tied to the White House.
Hinting at secret tapes, Trump warns ousted Federal Bureau of Investigation director
White House officials said Trump is considering almost a dozen candidates to replace Comey, including Texas Sen. And at his NY office during the campaign, workers believed Trump had a system set up to record phone calls.
President Trump's tweet came today after "Morning Joe" host Mika Brzezinski excoriated various White House spokespeople, including Kellyanne Conway, Sarah Huckabee Sanders and Sean Spicer, saying that they have been proven "not credible" in repeated appearances. In an interview on Thursday, he insisted he always meant to fire the Federal Bureau of Investigation director, undercutting the initial explanation that he acted on the recommendation of top officials who criticised Comey's handling of a probe into Hillary Clinton's emails. And at that time, he told me: "'You are not under investigation'".
At a congressional hearing in March, Comey refused to answer a question from Rep. Eric Swalwell (D-Calif.) as to whether or not Trump himself was under investigation by the FBI. On Capitol Hill, acting FBI Director Andrew McCabe strongly disputed the White House's assertion that Comey had been fired in part because he had lost the confidence of the FBI's rank-and-file.
Previous presidents have made a public show of staying out of legal matters, so as not to appear to be injecting politics.
Trump and Comey's relationship was strained early on, in part because of the president's explosive and unsubstantiated claims that Barack Obama wiretapped Trump Tower. Mr. Comey again replied that he would give him "honesty" and did not pledge his loyalty, according to the account of the conversation.
"The FBI has been in turmoil".
McCabe also told Collins he wasn't familiar with any meeting Comey had with Deputy Attorney General Rod Rosenstein to request more resources for the Russian Federation investigation.