The emails add to mounting controversy about possible ties between the Trump campaign and Russia ahead of the 2016 US election.
In June last year, Donald Trump Junior received an email saying a Russian government lawyer was offering damaging information about his father's Democratic rival for president.
An email from publicist Rob Goldstone, who brokered the meeting, said the documents would incriminate Hillary Clinton, regarding her dealings with Russia.
It said the documents would be "very useful to your father."
Donald Trump Junior replied within minutes, saying, "If it's what you say, I love it, especially later in the summer."
Mr Goldstone said the information offered was, as he put it, "part of Russia and its government's support for Mr Trump."
Donald Trump Junior has made the email exchange public on his Twitter page after The New York Times said it planned to write about the contents and sought his comment.
But the Russian lawyer at the centre of the controversy, Natalia Veselnitskaya, has told NBC television in the United States she had no ties with the Russian government.
She says her intent was merely to discuss US sanctions, not the campaign.
"When it was suggested I meet with Donald Trump Junior, I met him in a private situation. It was a private meeting, not related at all to the fact that he was the son of the candidate."
The Trump campaign has steadily denied any meetings of significance with the Russians.
Donald Trump Junior says he never told his father about the meeting because "there was nothing to tell."
But the top-ranking Democrat on the House Intelligence Committee, Adam Schieff, says the new information should have been reported to authorities.
"Rather than report this overture by the Russian government to provide damaging information to intervene in the presidential election in a way to help his father, neither the President's son nor the campaign reported this information to the FBI."
A legal analyst for CNN, Laura Coates, says the emails could be very important for US authorities investigating any collusion between Russia and the Trump campaign.
"It's a huge smoking gun. For the very first time in this counterespionage probe that has been fixated on the Trump campaign's potential involvement with any Russian official and potential collusive activity, you now have somebody confirming -- and it's Donald Trump Junior himself, a member of the campaign, confirming -- that there was, in fact, contact with Russian officials, or an attempt to secure information and a coordinated attack, if you will, against the opponent, Hillary Clinton."
In a statement read by White House spokeswoman Sarah Huckabee Sanders, President Donald Trump says he is standing by his oldest son.
"I've a quick statement that I will read, from the President: 'My son is a high-quality person, and I applaud his transparency.' And, beyond that, I'm going to have to refer everything on this matter to Don Junior's counsel and outside counsel and won't have anything else to add beyond that."
But Republican senator Lindsey Graham says the offer from a foreign government should never have been accepted.
"On its face, this is very problematic. We cannot allow foreign governments to reach out to anybody's campaign and say, 'We'd like to help you.' That is a non-starter."
Democratic senators are calling for the President's son to testify before a congressional committee investigating possible Russian interference in the election.
One of the senators, Chuck Schumer, says it is important.
"We need Donald Trump Junior and other campaign associates to turn over any and all documents and electronic communications that investigators ask for. And we need public testimony under oath (from) Donald Trump Junior, who has changed his story repeatedly."
Democratic senator Tim Kaine, Hillary Clinton's vice-presidential nominee, says there could be serious criminal charges.
"Nothing is proven yet, but we're now beyond obstruction of justice, in terms of what's being investigated. This is moving into perjury, false statements and even into, potentially, treason."
Share
