The US Court of International Trade on Thursday ruled in favour of small businesses that challenged the tariffs, which took effect on February 24.
The ruling was 2-1, with one judge saying it was premature to grant victory to the small business plaintiffs.
The small businesses had argued the new tariffs were an attempt to sidestep a landmark Supreme Court decision that struck down the Republican president's 2025 tariffs imposed under the International Emergency Economic Powers Act.
In his February order, Trump invoked Section 122 of the Trade Act of 1974, which allows for duties for up to 150 days to correct serious "balance of payments deficits" or head off an imminent depreciation of the dollar.
Thursday's court ruling found the law was not an appropriate step for the kinds of trade deficits that Trump cited in his February order.
"This decision is an important win for American companies that rely on global manufacturing to deliver safe and affordable products. Unlawful tariffs make it harder for businesses like ours to compete and grow," said Jay Foreman, CEO of toymaker Basic Fun!
"We are encouraged by the court's recognition that these tariffs exceeded the President's authority. This ruling brings needed clarity and stability for companies navigating global supply chains," he said in a statement.
The Trump administration had argued that a serious balance-of-payments deficit existed in the form of a $US1.2 trillion ($A1.7 trillion) annual US goods trade deficit and a current account deficit of four per cent of GDP.
But some economists and trade lawyers argue the U.S. is not on the cusp of a balance-of-payments crisis, making the new duties vulnerable to a legal challenge.
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