The New York Times said the USA president had told top Russian officials Mr Comey's sacking had relieved "great pressure" on him.
The New York Times, citing a document summarising last week's meeting, says he called Mr Comey a "real nut job".
Russia has denied it interfered in the presidential election, and the White House rejects allegations that members of the Trump campaign colluded with Russian officials in the matter. I have said that all day long based on my conversations with White House officials, and they continue to say as much as well.
An unnamed American official read out a summary of the meeting to the Times.
Contacted by the Times, White House press secretary Sean Spice did not deny Trump had made the statements, saying Comey's "grandstanding and politicizing" of the Russian Federation probe had put "unnecessary pressure on our ability to engage and negotiate with Russian Federation".
The White House has not disputed the language used in the meeting at the Oval Office between Mr Trump, Russian Foreign Minister Sergei Lavrov and the Russian Ambassador Sergei Kislyak.
Rosenstein declined to discuss the timing of the memo and who had asked him to write it, saying the memo and its role in Comey's firing were likely to be part of the investigation, which will now be led by Mueller, whom Rosenstein appointed special counsel Wednesday. So they know perfectly well he's not insane and not a nut job.
Donald Trump reportedly told his Russian guests that firing Mr Comey had relieved the "great pressure" his administration was under because of the Russia investigation. That's because they aren't idiots.
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This comes on the heels of Comey's recent unexpected dismissal as Federal Bureau of Investigation director by President Trump, which raised a lot of eyebrows in the government and the media, and the ethics of Trump's decision has been widely questioned by the public.
On Friday, deputy attorney general Rod Rosenstein told Congress he stands by a memo he wrote bluntly criticising Mr Comey.
Trump's May 9 decision of fire Comey has set off a series of events and fueled speculation surrounding the Russian Federation investigation.
The White House first pointed to the memo as a reason for Comey's firing. Mr. Trump later acknowledged that he planned to fire Mr. Comey all along, regardless of Mr. Rosenstein's recommendation.
So far, President Trump's former national security adviser, Michael Flynn, and former campaign manager, Paul Manafort, have been the public centers of the investigation.
Comey was sacked on May 9.
Speaking to NBC News after Comey's firing, Trump mentioned the Russian Federation investigation when explaining why he fired Comey, whom he called a "showboat" and a "grandstander".
In Rosenstein's meetings with lawmakers, he reiterated the scathing critique laid out in his memo concerning Comey's handling of the investigation into Clinton's use of a private email server, including Comey's July news conference announcing that the FBI would not recommend charges.