The US President insists his son is innocent of any wrongdoing and the investigation into links with Russia is what he again is calling "the greatest witch-hunt in political history."
Donald Trump has taken to Twitter to defend his son as "open, transparent and innocent."
The comments come after Donald Trump Junior released emails showing he met with a Russian lawyer, hoping to get information incriminating Hillary Clinton in last year's election.
Mr Trump Junior then appeared in a television interview on Fox News to defend his actions, but admitted he would "have done things a little differently" in retrospect.
The President has been quick to praise his son's interview, messaging on Twitter that Donald Trump Junior is a victim of the "greatest witch-hunt in political history."
"My son Donald did a good job last night. He was open, transparent and innocent. This is the greatest witch-hunt in political history. Sad!"
And on the ongoing investigation by authorities and journalists into his presidential campaign's links with Russia, he tweeted:
"Remember, when you hear the words 'sources say' from the Fake Media, oftentimes those sources are made up and do not exist."
The emails released by Mr Trump Junior came after The New York Times approached him for comment, saying it was preparing to publish a story revealing the emails.
They are the most concrete evidence yet that campaign officials welcomed Russian help to win the 2016 presidential election.
The emails do not appear to provide evidence of illegal activity, but legal analysts say Mr Trump Junior could be in trouble if investigators find he aided a criminal action.
That could involve hacking into Democratic computer networks or that he violated campaign-finance laws by accepting gifts from foreign entities.
Russian foreign minister Sergey Lavrov says he is surprised Mr Trump Junior has been accused of interacting with a Russian lawyer.
Mr Lavrov claims he first heard about the meeting between President Trump's son and Natalia Veselnitskaya on Monday.
He says it has created unnecessary fuss.
"It is so bizarre, because what problem are we talking about? What threat are we talking about if there is an interaction between a client and a lawyer? I didn't know about that before. I heard about it yesterday from the news, and today, this morning, once again I turned on the TV, and on Western channels they keep discussing it. It is amazing how you can make a buzz from nothing."
In Washington, Donald Trump's pick to head the country's FBI, Christopher Wray, has appeared before a US Senate Judiciary Committee confirmation hearing.
Mr Trump controversially fired his predecessor, James Comey, in May over his investigation into links between Russia and the Trump campaign.
Facing questions on the FBI's independence, Mr Wray said he would refuse to pledge loyalty to Mr Trump and promised to quit if the President told him to do something unlawful.
He said he also rejected the President's description of the investigation into Russian election meddling as a "witch-hunt."
"If I am given the honour of leading this agency, I will never allow the FBI's work to be driven by anything other than the facts, the law and the impartial pursuit of justice. Period, full stop."
There have been reports the White House is in chaos as the latest scandal widens.
But Mr Trump quickly denied that, tweeting: "The White House is functioning perfectly, focused on healthcare, tax cuts/reform and many other things. I have very little time for watching TV."
