Investigators are probing his father's campaign over alleged collusion with Moscow in last year's election.
Donald Trump Jr was grilled behind closed doors in Congress on Wednesday about his contacts with Russia as pressure builds on his father over alleged collusion with Moscow in last year's election.
His June 2016 meeting with Russian lawyer Natalia Veselnitskaya at the height of the presidential campaign has made him a key figure in investigations into the Trump campaign's possible ties to the Kremlin.
A separate 2016 meeting with Alexander Torshin, a senior Russian politician and central banker close to President Vladimir Putin, has added to the scrutiny of President Trump's eldest son.
He entered and exited the House Intelligence Committee hearing from a back door, avoiding the media, and nothing was immediately released about the discussions.
But according to CNN, the Veselnitskaya meeting, and how the White House tried to explain it away as a discussion about US adoption of Russian orphans, was one focus of the questioning.
NBC News reported on Tuesday that, in recent written testimony to the Senate Intelligence Committee, Veselnitskaya said Trump Jr had asked her in the June 2016 meeting for evidence of illegal donations to the foundation of former president Bill Clinton, the husband of Trump's Democratic election rival Hillary Clinton.
DIRT ON CLINTONS
But Veselnitskaya said she hadn't offered anything like that and had only met to discuss the US Magnitsky Act used to place sanctions on high-level Russians.
"Today, I understand why it took place to begin with and why it ended so quickly with a feeling of mutual disappointment and time wasted," Veselnitskaya wrote of the meeting, according to NBC.
She also said that when he found that she did not have any dirt on the Clintons, Trump Jr appeared to lose interest.
In May last year Trump Jr also met Torshin at a US National Rifle Association event.
Trump Jr's lawyer Alan Futerfas said last month that there was only "small talk" between them. But emails have shown Torshin trying to arrange a meeting with then-candidate Trump, an overture that was turned down by Jared Kushner, Trump's son-in-law.
Those contacts and others not previously reported publicly by the Trump campaign are important to the investigation by special prosecutor Robert Mueller, who is looking at allegations of collusion and also possible obstruction of justice by President Trump and people around him.
Mueller has charged four people so far, including two — George Papadopoulos, a campaign foreign policy advisor who maintained Russian contacts, and former White House national security advisor Michael Flynn — with lying to investigators.