He delivered the threatened duty in a social media post on Wednesday, hours after agreeing to a two-week ceasefire with Tehran.
Trump's Truth Social post did not specify which legal authority he would invoke to impose such tariffs, as the Supreme Court in February struck down his use of the International Emergency Economic Powers Act to impose broad global tariffs, prompting a lower court to order refunds of some $US166 billion ($A235 billion) collected over the course of a year.
The 1977 IEEPA law has been used extensively for decades to back financial sanctions against Iran, Russia and North Korea, but the court ruled Trump overstepped his authority in using it to impose trade tariffs.
"A Country supplying Military Weapons to Iran will be immediately tariffed, on any and all goods sold to the United States of America, 50 per cent, effective immediately, There will be no exclusions or exemptions! President DJT," Trump wrote.
He did not name any countries that could face punitive tariffs. China and Russia have helped Iran build military capacity to counter US and Israeli pressure, supplying missiles, air-defence systems and technology intended to bolster deterrence.
But that support appeared capped during the US-Israeli attacks on Iran. Both Beijing and Moscow have denied supplying any weapons recently, although allegations against Moscow have persisted.
Reuters reported in February, prior to the first US and Israeli strikes on Iran, that Tehran was considering a purchase of supersonic anti-ship cruise missiles from China.
Reuters also reported in March that China's top semiconductor maker, SMIC , has sent chipmaking tools to Iran's military, according to two senior Trump administration officials.
Trump does have active Section 301 unfair trade practices tariffs on Chinese goods from his first term to which he may be able to add duties and similar pending cases related to excess industrial capacity and China's compliance with a 2020 trade deal. But these would require a public notice period before they could take effect.
Trump also may be able to invoke Section 232 of the Cold War-era Trade Expansion Act of 1962, which allows sector-specific tariffs to protect strategic domestic industries on national security grounds, but using this law would require a new months-long investigation and public comments.
Any new tariffs on Chinese goods would inflame tensions with Beijing as Trump prepares for a planned trip to Beijing to meet with Chinese President Xi Jinping to firm up a delicate trade truce between the world's two largest economies.
US imports from Russia, one of the only countries not subject to Trump's now-cancelled "reciprocal" tariffs, jumped 26.1 per cent to $US3.8 billion ($A5.4 billion) in 2025.
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