The US President was speaking to the Sunday Times ahead of his arrival in the UK on Monday for a state visit, the BBC reported.
"If you don't get the deal you want, if you don't get a fair deal, then you walk away," he told the daily.
Trump suggested that the UK should not pay the 39 billion pounds ($49 billion) Brexit "divorce" bill laid out in the current withdrawal agreement.
"If I were them I wouldn't pay $50 billion, it's a big number," he said.
To compensate for lost trade with the EU, Trump vowed to "go all out" to secure a free trade deal between the UK and US within months of Britain leaving the bloc.
The first day of Trump's state visit to the UK is set to include a private lunch with the Queen, tea with the Prince of Wales and the Duchess of Cornwall, and a state banquet at Buckingham Palace.
Trump also said that the UK should include Brexit Party leader Nigel Farage in talks with the EU, describing him as "a very smart person".
His comments came after he said former Foreign Secretary Boris Johnson would be an "excellent" Conservative Party leader to replace outgoing Prime Minister Theresa May.
( With inputs from IANS )