The US president announced the measures on Twitter, which are widely seen as a new front in his trade spat with much of the rest of the world as he seeks to protect US jobs.
He tweeted: “Brazil and Argentina have been presiding over a massive devaluation of their currencies. which is not good for our farmers.
“Therefore, effective immediately, I will restore the Tariffs on all Steel & Aluminum that is shipped into the US from those countries.”
Mr Trump also resumed his hostilities against the US central bank – urging the Federal Reserve to further cut interest rates and weaken the dollar, thus making US-produced goods more competitive abroad.
He said: “Reserve should likewise act so that countries, of which there are many, no longer take advantage of our strong dollar by further devaluing their currencies.
“This makes it very hard for our manufactures & farmers to fairly export their goods. Lower Rates & Loosen – Fed!”
Both South American nations were among US allies that Trump had exempted from steel and aluminium tariffs in March 2018.
Brazil’s economy is the ninth largest in the world, but it has been struggling to gain traction amid surging unemployment and a lack of structural reforms under President Jair Bolsonaro, who took office in January.
Its currency, the real, hit record lows versus the dollar last week.
Argentina also has a new government – set to assume power next week – as its currency, the peso, sags in a resurgent debt crisis that has prompted a series of currency controls.
By far the biggest battle for the US is the trade war with China that has seen tit-for-tat tariffs applied to billions of dollars worth of each other’s goods since July 2018.
Relations between the world’s two biggest economies worsened last week despite several rounds of positive negotiations – setting back hopes of a so-called “phase one” trade deal to cool tensions.
The US is currently slated to raise tariffs against Beijing in mid-December should progress continue to lag.
Mr Trump is also pursuing a trade battle with the EU and imposed, in October, a round of tariffs on EU-made goods as part of the bloc’s battle with the US over state subsidies to Airbus of Europe and US-based Boeing.