Trump's tweet also urged the Federal Reserve to lower interest rates so countries "no longer take advantage of our strong dollar".
In a series of tweets Monday, the president pointed out that both countries have been "presiding over a massive devaluation of their currencies".
President Trump is reimposing steel and aluminum tariffs on Brazil and Argentina. Trump accused both countries of manipulating their currencies to hurt USA farmers, and he paired his action with criticism of the Federal Reserve, notes Bloomberg, which sees the morning moves as significant.
This surprise announcement came as investors paid close attention to the main trade war - between the USA and China. "This makes it very hard for our manufactures & farmers to fairly export their goods".
The South American nation's steel accounts for about 3.5 percent of the 110 million tons consumed in the USA a year.
In Argentina, investors are on the sidelines waiting for a new direction to the economy, which will be set by president-elect Alberto Fernandez when he takes office next week.
Trump has repeatedly criticized Fed chairman Jerome Powell for not cutting interest rates as much as the Republican president would like. But it also signaled that it planned no further interest rate cuts absent clear evidence of a worsening USA economic outlook.
After a brief spike in steel prices, USA steel producers have struggled since the Trump administration put tariffs into place a year ago. Brazil has been stepping up as a major supplier of soybeans to China while Beijing cut back on its USA farm purchases in retaliation for the tariffs.
Domestic demand has slumped along with the energy sector as drillers pull back on purchases of steel pipe. Fed officials believe that the USA economy should grow around 1.8 percent a year, near its current pace, while Trump believes the US economy should be growing much faster. It has laid off workers and shut down some of its blast furnaces.
AP reporter Mauricio Savarese contributed to this story from Sao Paulo.