Trump continued with his tweets overnight, railing against countries that he said have trade surpluses with the United States, even as he was preparing for a crucial one-on-one summit with North Korean dictator Kim Jong-un in Singapore.
He called Mr Trudeau "dishonest & weak" after Mr Trudeau said at a news conference that Canada would retaliate for new USA tariffs.
Trump had earlier lashed out at Canadian leader Justin Trudeau for what he said were "false statements" at a news conference and said the U.S. would not endorse the G7 communique.
"And I said, 'push him around?'" Trump said, describing his reaction to the press conference.
In a string of angry tweets, Trump said the United States paid "close to the entire cost of Nato" to help protect countries that "rip us off on trade".
"I know my colleagues are hearing from numerous businesses and manufacturers across the country very similar stories, that this trade dispute is probably two weeks away from affecting Canadians in a very real way", he said.
Trump has consistently railed against what he claims are unfair trade practices by some of America's biggest trade partners, including Canada.
"As acknowledged by CBC/Radio Canada, 'Canada levies a tariff of 270 percent on milk, 245 percent on cheese and 298 percent on butter in an effort to keep US and other foreign dairy imports out'".
Unbowed, Mr Trump tweeted anew on Monday morning from Singapore: "Fair Trade is now to be called Fool Trade if it is not Reciprocal".
He told reporters that imposing retaliatory measures "is not something I relish doing" but that he wouldn't hesitate to do so because "I will always protect Canadian workers and Canadian interests".
Mr Trump's advisers said the USA president believed he had been stabbed in the back by Mr Trudeau over United States tariffs.
Former Conservative cabinet minister James Moore, a member of the government's advisory group on NAFTA, hailed Trudeau's approach, refusing to react to "the noise, the bluster, the Twitter, the emotional outbursts". Among other things, she said the government should be preparing to keep pace with corporate tax cuts and tax breaks south of the border.
Trump was infuriated by the news conference and abruptly withdrew USA support for the summit's relatively anodine closing statement, a boilerplate invocation of the importance of multilateral trade rules.
On Monday, MPs in the House of Commons approved a motion denouncing Trump's name-calling tirade and endorsing Trudeau's decision to stand his ground against US tariffs and tweeted presidential threats. "Chief among them are Canada's extraordinarily high tariffs on American dairy products".
Former U.S. Ambassador Bruce Heyman, who served under former U.S. President Barack Obama, has also backed Trudeau. These countermeasures amount up to 16.6 billion Canadian dollars (about 12 billion US dollars) in imports of steel, aluminum, and other products from the Unites States, and are set to come into effect on July 1 this year.