The Labour deputy leader spoke out after the BBC, The Guardian and The Daily Mail were among a raft of news organisations banned from a White House press briefing.
Mr Watson, also shadow culture secretary, told ITV’s Peston on Sunday the ban was “outrageous”.
He said: “If you start attacking the freedom of the press like the most powerful man in the world is, then we really are in trouble.
“Theresa May has obviously formed a very close relationship with Donald Trump.
“I think she needs to get the Daily Mail, The Guardian and the BBC back in the news room of the United States president, because that’s the only way that democracy can hold powerful people to account.”
Shortly after President Trump gave a speech attacking the media as the “enemy of the people”, his press secretary Sean Spicer restricted multiple media organisations from entering the daily briefing.
CNN, Buzzfeed and The New York Times, of which Mr Trump has been highly critical, were also denied access.
Mr Watson also defended a £500,000 donation he received from Max Mosley.
The former Formula One boss is the primary financial backer of the Impress press regulator, having previously been the victim of a newspaper sting involving images of him at a sex party
“I think anyone that knows me knows that I could never be bought,” Mr Watson said.
“And I think anyone that knows me knows that I’ve had very strong views on reform of the press before the donation from Max Mosley was given to me.
“We got to know each other and became friends through our joint commitment to media reform and that’s not going to change, no matter who gives me a donation.”