The US Supreme Court has ruled that Donald Trump’s sweeping tariffs are illegal, in a landmark rebuke to the economic centrepiece of the US president’s second term.

America’s top court ruled in a 6-3 vote on Friday that Trump exceeded his authority in using the International Emergency Economic Powers Act to impose tariffs on dozens of US trading partners.

In its ruling, the Supreme Court said: “Our task today is to decide only whether the power to ‘regulate . . . importation,’ as granted to the President in IEEPA, embraces the power to impose tariffs. It does not.”

The decision marks the first major defeat for Trump at the Supreme Court, after it handed the president several wins in previous cases including on the deportation of migrants.

Chief Justice John Roberts wrote the ruling for the majority. Justices Clarence Thomas, Samuel Alito and Brett Kavanaugh dissented.

The case against the administration was brought by groups of American businesses, joined by 12 US states, that argued they had been harmed by the tariffs.

Trump returned to the White House last year vowing to use tariffs to remake a global trade order that he claimed had “ripped off” the US for decades.

But in its ruling, the court said: “In IEEPA’s half century of existence, no president has invoked the statute to impose any tariffs, let alone tariffs of this magnitude and scope.”

Trump announced his tariff regime on “liberation day” last April, sparking weeks of turmoil in financial markets and alarming US allies. 

Although he has since backed away from imposing some of the most severe duties, the US ended 2025 with an effective tariff rate of more than 10 per cent — the highest since the second world war.

The ruling drew a muted reaction from investors, with the US dollar index briefly dropping before recovering. The S&P 500 and Nasdaq Composite remained modestly higher.

Jason Borbora-Sheen, a portfolio manager at Ninety One, said the response indicated that the outcome was “largely expected . . . and the administration has had a while to plan their reaction”.

Stock markets have recovered since “liberation day” to hit record highs, but polls indicate that many Americans think the tariffs are hurting the country’s economy.

The US president did not immediately respond to the decision. A White House spokesperson did not immediately respond to a request for comment.