Trump signed paperwork enacting tariffs of 25 percent on steel and 10 percent on aluminum during a hastily arranged event at the White House.
The US would offer temporary relief to Canada and Mexico for 30 days, the Washington Post reported citing senior US officials, breaks that could be extended depending on progress in the ongoing North American Free Trade Agreement negotiations.
Senate Minority Leader Chuck Schumer (D-N.Y.), in a speech delivered on the Senate floor, said Trump should "rethink these tariffs and focus his policy more directly at China and countries that ship cheap Chinese steel to the U.S".
A separate proclamation was signed by Trump for aluminium, utilizing virtually the same language except for differences in grades covered by the orders. But the White House has repeatedly undermined its own legal case, including by intimating that the tariffs would be lorded over Canada and Mexico as some kind of negotiating tool to extract NAFTA concessions.
On Friday morning, Mr Trump talked up the US-Australian relationship and indicated Australia would be an ally that would not face the tariffs.
The White House noted that more than 50,000 U.S.jobs have been lost in the steel industry and 40,000 in aluminum since 2000. "Under current circumstances, this tariff is necessary and appropriate to address the threat that imports of steel articles pose to the national security".
The tariffs would go into effect in two weeks and in the meantime, the USA could expect to see a number of countries requesting to be granted exemptions.
Canadian Trade Minister Francois-Phillippe Champagne said his country would not accept any duties or quotas from the US.
US President Donald Trump has signed off on deeply contentious trade tariffs, swatting aside warnings of a global trade war and protests from allies in Europe and at home.
But Trump also seemed to leave the door open for certain forms of compromise, saying the United States would remain "flexible" toward its trading allies.
Tonight, Speaker Paul Ryan said he feared there could be "unintended consequences" after the introduction of the tariffs.
They will come into force in 15 days' time, but Canada and Mexico - who are now renegotiating the North American Free Trade Agreement (NAFTA) with the U.S. - will initially be exempt.
"The European Union, wonderful countries who treat the USA very badly on trade, are complaining about the tariffs on steel and aluminum". "The US's "abuse" of national security concerns as a reason for implementing the measures will lead to "a serious attack on normal global trade order", the release read".