Trump Jr. hired New York lawyer Alan Futerfas, who specialises in criminal defence.
Futerfas would not say when he was retained or whether he played any part in the statements Trump Jr. made over the weekend about a meeting with a Russian lawyer.
Trump Jr. met the Russian, Natalia Veselnitskaya, in June 2016 during the presidential election campaign at Trump Tower in New York.
He has said he agreed to meet Veselnitskaya, described by the New York Times as having links to the Kremlin, after being promised damaging information about Democratic presidential candidate Hillary Clinton.
Trump’s son-in-law Jared Kushner and Trump’s then-campaign chairman, Paul Manafort, also attended, the Times said. It called the encounter the first confirmed private meeting of members of Donald Trump’s inner circle with a Russian national.
Congressional committees and a federal special counsel, Robert Mueller, are investigating whether Russia interfered in the election and colluded with the Trump campaign.
Moscow denies interfering, and Trump, who became president on January 20, says there was no collusion.
Futerfas told Reuters: “I look forward to assisting Donald Jr. and, quite frankly, there is nothing to all of the media buzz about the June 9th, 2016 meeting. That will be proven to be the case.”
Trump Jr tweeted on Monday: “Happy to work with the committee to pass on what I know.”
Earlier on Monday, a Republican member of the US Senate Intelligence Committee, Susan Collins, called on Trump Jr. to testify before the panel, which is looking into accusations of Russian meddling in the election.
“Our intelligence committee needs to interview him and others who attended the meeting,” Senator Susan Collins told reporters at the US Capitol.
Aides to leaders of the Senate and House intelligence committees declined to comment about whether Trump Jr. would be called to testify.
Allegations of the Trump campaign’s ties to Russia have cast a shadow over the Republican president’s first months in office.
White House spokeswoman Sarah Sanders said there was nothing inappropriate about the meeting.
“Don Junior took a very short meeting from which there was absolutely no follow-up,” Sanders told reporters.
But Republican Senator Pat Toomey said it was not appropriate for presidential campaigns to solicit or accept negative information about a rival from a Russian.
“It encourages countries to come in and undermine our democratic process. And these countries have great capabilities to do that sort of thing,” he told MSNBC.
In Moscow, Kremlin spokesman Dmitry Peskov told reporters on Monday that the Kremlin did not know the identity of the Russian lawyer.
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