The Manhattan-based Court of International Trade said the US Constitution gives the US Congress exclusive powers to regulate commerce with other countries that are not trumped by the president's emergency powers to safeguard the US economy.
"The court does not pass upon the wisdom or likely effectiveness of the President's use of tariffs as leverage. That use is impermissible not because it is unwise or ineffective, but because (federal law) does not allow it," a three-judge panel said in the decision.
The Trump administration minutes later filed a notice of appeal.
The ruling came in a pair of lawsuits, one filed by the nonpartisan Liberty Justice Centre on behalf of five small US businesses that import goods from countries targeted by the duties and the other by 13 US states.
The companies - which range from a New York wine and spirits importer to a Virginia-based maker of educational kits and musical instruments - have said the tariffs will hurt their ability to do business.
The White House and lawyers for groups that sued did not immediately respond to requests for comment.
Stephen Miller, a White House deputy chief of staff and one of Trump's lead policy advisers, rebuked the court in a brief social media post, writing: "The judicial coup is out of control."
At least five other legal challenges to the tariffs are pending.
Oregon Attorney General Dan Rayfield, a Democrat whose office is leading the states' lawsuit, called Trump's tariffs unlawful, reckless and economically devastating.
"This ruling reaffirms that our laws matter, and that trade decisions can't be made on the president's whim," Rayfield said in a statement.
Trump has claimed broad authority to set tariffs under the International Emergency Economic Powers Act (IEEPA), which is meant to address "unusual and extraordinary" threats during a national emergency.
The law has historically been used to impose sanctions on enemies of the US or freeze their assets. Trump is the first US president to use it to impose tariffs.
In imposing the tariffs in early April, Trump called the trade deficit a national emergency that justified his 10 per cent across-the-board tariff on all imports, with higher rates for countries with which the United States has the largest trade deficits, particularly China.
Many of those country-specific tariffs were paused a week later. The Trump administration on May 12 said it was also temporarily reducing the steepest tariffs on China while working on a longer-term trade deal. Both countries agreed to cut tariffs on each other for at least 90 days.
Trump's on-and-off-again tariffs, which he has said are intended to restore US manufacturing capability, have shocked US financial markets.
Tariffs must typically be approved by Congress. The US president imposed tariffs on most countries in the world at one point, sending markets reeling.
with AP
