Donald Trump has signed two proclamations imposing 25% tariffs on all steel and aluminium imports to the US.
A proclamation is a form of presidential directive to government officials, but they do not carry the force of law, as an executive order would.
However the White House has said the tariffs will take effect from 4 March.
"This is a big deal," Mr Trump said in the Oval Office as he announced the tariffs. "The beginning of making America rich again."
He added: "We were being pummelled by both friend and foe alike."
Read more
Analysis: Trump's steel and aluminium tariffs target a deeper issue
Trump issues Gaza ceasefire ultimatum to Hamas
Analysis: The country that could benefit most from Trump tariffs
The proclamations mean the president has now removed the exceptions and exemptions from his 2018 tariffs on steel to allow for all imports of the metal to be taxed at 25%.
The new tariff on aluminium is also much higher than the 10% duty he imposed on the material in his first term.
The tariffs are part of an aggressive push by Mr Trump to reset global trade, as he claims that price hikes on the people and companies buying foreign-made products will ultimately strengthen domestic manufacturing.
Outside economic analyses suggest the tariffs would increase costs for the factories that use steel and aluminium, possibly leaving US manufacturers worse off.
European Commission president Ursula von der Leyen responded: "Unjustified tariffs on the EU will not go unanswered -
they will trigger firm and proportionate countermeasures".
Canada, the largest source of steel imports to the US, also criticised the move.
Candace Laing, CEO of the Canadian Chamber of Commerce, said Mr Trump was destabilising the global economy.
"Today's news makes it clear that perpetual uncertainty is here to stay," she said.
Trump's war of tariffs
Mr Trump's proclamations come days after the US imposed a 10% tariff on all goods imported from China.
In return, China imposed 10% tariffs on American crude oil, agricultural machinery, large-displacement cars and pickup trucks.
There will also be 15% tariffs on coal and liquefied natural gas from the US.
US plans to impose 25% tariffs on Mexico and Canada were paused after agreements were reached on border security.
Mexico's president said she was sending 10,000 National Guard troops to the US border immediately in return for a tariff delay.
Mr Trump said the Mexican soldiers would be "specifically designated" to stop the flow of fentanyl into the US, as well as illegal migrants.
Meanwhile, Canada's prime minister Justin Trudeau said almost 10,000 frontline personnel "are and will be working on protecting the border".
He added that his country was appointing a "fentanyl czar", drug cartels would be listed as terrorists, and there would be "24/7 eyes on the border".
(c) Sky News 2025: Trump imposes 25% tariffs on all steel and aluminium imports