"Unfortunately, we have a president, literally, for whom the truth, the whole truth and nothing but the truth is three different things, and you don't even know what they are," Kerry said on the HBO late-night show. "He's got the maturity of an eight-year-old boy with the insecurity of a teenage girl. That's just who he is."
"He is the first president that I know of who spends more time reading his Twitter 'likes' than his briefing books or the constitution of the United States," he continued.
Earlier this week, Trump tweeted that Kerry's meetings with the "hostile" Iranian regime were "illegal," and that the Obama administration member was "BAD."
Kerry told Maher that it's not out of the ordinary for a former politician to meet with other officials.
"Well Henry Kissinger, for 40 years, has been traveling to Russia, traveling to China -- talks with the leaders. There's absolutely nothing unusual about it," he said.
Kerry fired back at POTUS on Twitter, too, saying he should be "more worried" about Paul Manafort meeting with Special Counsel Robert Mueller.
John Kerry had illegal meetings with the very hostile Iranian Regime, which can only serve to undercut our great work to the detriment of the American people. He told them to wait out the Trump Administration! Was he registered under the Foreign Agents Registration Act? BAD!
Mr. President, you should be more worried about Paul Manafort meeting with Robert Mueller than me meeting with Iran's FM. But if you want to learn something about the nuclear agreement that made the world safer, buy my new book, Every Day Is Extra: https://t.co/DKjc33Kvvu https://t.co/cesltkt0zW
It was announced on Friday that Manafort -- Trump's former presidential campaign chairman -- struck a cooperative plea deal with prosecutors in exchange for some of the charges against him being dropped. He pleaded guilty in court Friday to one count of conspiracy against the US and one count of conspiracy to obstruct justice due to attempts to tamper with witnesses.
"Soon you'll be hearing [Trump] say, 'That's the worst deal that's ever been made,'" Kerry told Maher. "It's gone from the 'Art of the Deal' to the art of the squeal."
"I've never seen anything quite like it," Kerry said. "The anonymous op-ed, the Woodward book, the chronology of everything that's been happening. It's tough. We've always relied on our democracy, getting better, being stronger when people tell the truth."
Although Kerry, Maher, and the late-night host's roundtable continuously ripped Trump Friday night, Maher brought some in some optimism for his viewers and said that "Fat Donny" might be meeting his downfall.
"The polls for him are going down," Maher said. "The looks on the faces at the people at his rallies -- I think the magic is gone. They're bored. He hasn't written new material. They have a look like, 'Man, it's a lot about you.'"