The order was effective from February 7.
The rate is to be determined by Secretary of State Marco Rubio.
Trump last month issued a threat of 25-per cent tariffs on any country that maintains business ties with Iran.
The two countries engaged in talks last week.
Such tariffs would affect trade with several nations like Germany, Turkiye, Russia and the United Arab Emirates.
Trump may modify the order if circumstances change, in response to retaliation, or if Iran or an affected country takes significant steps to address the national emergency and align with the United States on national security, foreign policy and economic matters, a White House fact sheet said.
Trump holds Iran accountable for its pursuit of nuclear capabilities, support for terrorism, ballistic missile development and regional destabilisation that endanger American security, allies and interests.
He recently deployed a massive armada to the region, urging Iran to come to the table and negotiate a fair, equitable deal with no nuclear weapons, or face even stronger consequences.
Fibre2Fashion (DS)