'Well, he's two-faced,' Trump said of him.
The president also canceled his final press conference at the NATO summit, which was scheduled for Wednesday afternoon, after the three world leaders were caught mocking him.
Trump has spent more than two hours answering questions from reporters during his various sit downs with fellow NATO leaders.
'I canceled the news conference. I've done so many,' he said during a luncheon with NATO leaders Wednesday, adding: 'I think that’s enough.'
He also wrote on Twitter: 'When today’s meetings are over, I will be heading back to Washington. We won’t be doing a press conference at the close of NATO because we did so many over the past two days. Safe travels to all!'
Trudeau, French President Emmanuel Macron and British Prime Minister Boris Johnson were caught on a hot mic at the Buckingham Palace NATO reception Tuesday evening, appearing to gossip about Trump.
Justin Trudeau, center, Emmanuel Macron, second right, and Boris Johnson, right, have all been caught on a hot mic at the Buckingham Palace NATO reception appearing to gossip about Donald Trump and an apparent impromptu lengthy press conference
Trump and Trudeau appeared to speak at a NATO leaders' meeting Tuesday morning
In the clip, which appears to be edited, Trudeau makes the most cutting comments, laughing about Trump's lengthy Q&A sessions with reporters and saying even the president's staff was shocked at some of his actions.
The president did not look amused when reporters asked him about the video during his meeting with German chancellor Angela Merkel.
'Honestly with Trudeau, he's a nice guy,' the president added after a long pause.
He then explained his theory on why Trudeau said what he did, arguing the prime minister mocked him because he gave Trudeau grief during their sit down on Tuesday about Canada not paying enough in fees to be a member of the NATO alliance.
'I find him to be a very nice guy but you know the truth is that I called him out on the fact that he's not paying 2 per cent and I guess he's not very happy about it,' Trump said.
'He's not paying 2 per cent, and he should be paying 2 per cent. It's Canada. They have money. And they should be paying 2 per cent. So I called him out on that, and I'm sure he wasn't happy about that, but that's the way it is. Look, I'm representing the U.S. And he should be paying more than he's paying. And he understands it. So I can imagine he's not that happy. But that's the way it is,' he added.
Trump and Trudeau appeared to speak at a photos session for NATO leaders on Wednesday morning but it's unclear what was discussed.
At their Tuesday meeting, Trump noted Canada is 'slightly delinquent' in its NATO payments but added: 'But they'll be okay. I have confidence.'
'Some are way below 1