"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 is scheduled to be sentenced next month in a separate case where he pleaded guilty to violating campaign finance laws to help Trump win the presidency by suppressing stories about Trump's alleged affairs. He also admitted that he lied on his taxes, and for those crimes he likely faces three to five years in prison, according to his earlier plea agreement.
Cohen admitted in federal court Thursday that Trump spoke with him more extensively about the proposed Trump Tower project in Moscow than he previously told Congress, pleading guilty to a charge brought by special counsel Robert Mueller.
Trump's former lawyer had previously said talks about the Moscow project ended in January 2016 just prior to the Iowa caucuses.
Cohen became the fifth Trump associate charged in the Russia investigation, joining former national security adviser Michael Flynn, senior campaign officials Paul Manafort and Ricks Gates as well as campaign adviser George Papadopoulos.
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.
In the past few months, Trump has repeatedly criticized people who cooperate with federal investigations. He has also spread an unproven and unsupported theory that Mueller's team is coercing people close to Trump to lie about him to bring down his presidency.
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. In Cohen's now-discredited testimony, he told lawmakers that he made the decision to terminate the deal on his own without consulting with Trump or any members of the Trump family.
However, Trump maintained that "there would've been nothing wrong if I did do it."
Trump also falsely said that the real estate proposal was "very public" and that "everybody knew about" the deal because "it was written about in newspapers" and elsewhere. But the deal was kept secret as it was negotiated. It only came to light after Trump took office, with most of the details being revealed in documents Cohen submitted to Congress in August 2017, which he also released to the public.
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."