"He's a weak person," Trump told reporters on the White House South Lawn before departing for Buenos Aires, Argentina.
"He was convicted with a fairly long-term sentence with things unrelated to the Trump Organization," Trump said, citing Cohen's legal issues with mortgages and the IRS.
Trump speculated that "what he's trying to do is get a reduced sentence."
Cohen admitted in federal court Thursday that Trump spoke with him more extensively about the proposed Trump Tower project in Moscow than Cohen previously told Congress.
Cohen, Trump's former lawyer, pleaded guilty Thursday to making false statements to Congress about the Russia investigation in a charge brought by special counsel Robert Mueller.
Cohen had previously said talks about the Moscow project ended in January 2016 just prior to the Iowa caucuses.
Trump later reiterated his disparagement of Cohen: "What he's trying to do -- because he's a weak person and not a very smart person."
"What he's trying to do -- and it's very simple, he's got himself a big prison sentence and he's trying to get a much lesser prison sentence by making up a story," Trump said.
Trump was emphatic that Cohen was "lying," but it is unclear what he believes Cohen is lying about.
"Michael Cohen is lying and he's trying to get a reduced sentence for things that have nothing to do with me," he said.
The President defended the deal for a possible Trump real estate venture in Moscow. The project, Trump said, "lasted a short period of time," adding that he decided not to do it because he wanted to focus on running for President.
However, he maintained that "there would've been nothing wrong if I did do it."
Asked why he worked with Cohen for so long, Trump said, "Because a long time ago he did me a favor. A long time ago he did me a favor."
Trump denied that Cohen is a threat to his presidency when asked by CNN's Kaitlan Collins.
"No," Trump said. "Not at all. I'm not worried at all about him."