"Yesterday showed no collusion, no obstruction", Mr Trump said in the White House Rose Garden during a news conference with the Romanian president, Klaus Iohannis.
The former Federal Bureau of Investigation chief was investigating whether the president or his campaign officials colluded with an alleged Kremlin plot to sway last year's U.S. presidential election in favour of Mr Trump.
During his testimony Comey also said that he spoke with the president nine times, and that he thinks he created a written record of each conversation.
And Comey said he's all for the president releasing any tapes - if they exist.
The president instead allowed surrogates to respond on Thursday, including his son, Donald Trump Jr., who posted dozens of tweets to his own account mocking Comey. I hardly know the man, it doesn't make sense'.
The way the Q&A unfolded, it certainly sounded as if Trump said he's prepared to testify under oath, which would raise the stakes on the investigation considerably.
"But in the meantime, no collusion, no obstruction".
"When it comes from the president, I took it as a direction", Comey said.
The Kasowitz accusations, made shortly after James Comey completed the public portion of his Senate testimony, if actively pursued, would necessarily shroud the Trump administration in a complex web of investigations, congressional hearings and court proceedings that would ultimately pull it into a black hole of legal destruction.
He said that he had carefully documented his interactions with Mr Trump because he anxious the president would misrepresent them, and accused the administration of spreading "lies, plain and simple" about the reasons for his firing.
The US president was asked by a journalist if he would be willing to give his version of events under oath.
"One hundred per cent", Mr Trump said. Pressed on the issue, he insisted he wasn't "hinting anything", before adding, "Oh you're going to be very disappointed when you hear the answer, don't worry".
The president also denied having asked for a pledge of loyalty, as Comey asserted during Thursday's much-anticipated testimony.
PRESIDENT DONALD TRUMP: I didn't say that.
Comey testified he had nine one-on-one conversations with Trump - and detailed each in memos just after they met for fear that the president would subsequently lie about their interactions.
Comey said he felt uncomfortable with the nature of his personal meetings with Trump, including a private dinner at the White House.
Comey's testimony Thursday was part of that investigation. Oregon Sen. Ron Wyden asked Comey.