The United Kingdom rolled out its two per cent digital services tax in 2020, a move that has been criticised by Trump and his predecessor Democrat Joe Biden.
"If they don't drop the tax, we'll probably put a big tariff on the UK," the president told reporters during a White House event.
His comments come ahead of a visit by Britain's head of state, King Charles, next week.
Trump said he believes the British monarch could help repair the Washington's relationship with the UK, which has become strained in recent months.
The tax targets companies whose worldwide revenues from digital activities exceed STG500 million ($A944 million), with more than STG25 million of the revenues from UK users.
Trump argued the laws, which have long been a source of tension in UK-US relations, targeted "top companies in the world".
"The UK did it, a couple of other people did it," he said.
"They think they're going to make an easy buck, that's why they've all taken advantage of our country."
The digital services tax went unchanged under the UK-US trade deal agreed in May 2025, despite being a point of discussion.
Asked how high the tariff would be, the US president said it would be "more than what they're getting" from the levy.
"What we'll do is we'll reciprocate by putting something on that's equal or greater than what they're doing," he said.
The latest remarks add to wider strains in UK-US relations, which have deteriorated after Prime Minister Keir Starmer ruled out British involvement in the conflict in the Middle East.
Earlier this month, Trump suggested the terms of the UK-US trade agreement brokered last year "can always be changed" in an interview with Sky News.
with PA
