He said his onetime foreign policy adviser George Papadopoulos was a "low level volunteer" and "liar". Mr Trump once called him an "excellent guy".
Papadopoulos, 30, this month pleaded guilty to lying to the FBI about his Kremlin-related contacts.
Two other ex-aides have been charged as part of the investigation into alleged Russian meddling in the US election.
What do ex-aides charges mean for Trump?
The other two, former campaign chief Paul Manafort and Rick Gates, appeared in court on Monday to deny charges including conspiracy and money laundering.
In Tuesday morning's tweets, Mr Trump attempted to deflect scrutiny on to the Democratic party, or his tax reform proposals.
He tweeted: "The Fake News is working overtime. As Paul Manaforts [sic] lawyer said, there was 'no collusion' and events mentioned took place long before he came to the campaign.
"Few people knew the young, low level volunteer named George, who has already proven to be a liar. Check the DEMS!"
Papadopoulos appears in a photo that Mr Trump tweeted of himself in March 2016 hosting a national security meeting with his foreign policy team.
In court documents released on Monday, federal investigators said Russian nationals had contacted Mr Papadopoulos to gain influence with the Trump campaign.
They offered "dirt" in the form of "thousands of emails" on Democratic presidential candidate Hillary Clinton in April 2016, according to an indictment.
That was two months before a trove of embarrassing emails related to Mrs Clinton's campaign were leaked after alleged Russian hacking of Democratic accounts.
Papadopoulos, who secretly pleaded guilty weeks ago to lying to the FBI about those contacts, has been co-operating with investigators for months.
Despite White House hopes to the contrary, there is no sign that Justice Department special counsel Robert Mueller's inquiry is near to completion.
Aaron Zelinsky, a prosecutor on Mr Mueller's team, told Papadopoulos' plea hearing this month: "There's a large-scale, ongoing investigation of which this case is a small part."
Russia's Foreign Minister Sergei Lavrov told reporters in Moscow on Tuesday that there was no evidence his country had interfered in the US elections.
"Everyone likes to talk but no one can present any facts," he said.
US intelligence has concluded that President Vladimir Putin of Russia tried to sway the outcome of the 2016 election in favour of Mr Trump.