Calling Prime Minister Justin Trudeau of Canada "very dishonest and weak", Trump said the US was withdrawing its endorsement of the G-7's communique, in part over what he called Trudeau's "false statements" about USA tariffs at a news conference.
At the news conference, Trudeau also said Canada will "move forward with retaliatory measures" on July 1 in response to the Trump administration's decision to impose tariffs on steel and aluminum imports from Canada, the European Union and Mexico.
Reacting to Trump's tweets, Trudeau's office said: "We are focused on everything we accomplished here at the summit".
American tariffs are in response to Canada's tariffs of "270% on diary" delivered by the U.S., he said.
Kudlow said the USA had spent the summit negotiating in "good faith" with Trudeau and the other assembled leaders, and that the U.S. had planned to sign the communique until Trudeau's news conference, which Kudlow called a "sophomoric play".
"International cooperation can not depend on being angry and on sound bites".
At the G-7 summit, Trump's spokeswoman, Sarah Huckabee Sanders, said the USA was "close to a deal" on NAFTA.
Mr Trump had earlier denied that the summit had been contentious, contradicting what one G7 official described as a bitter harangue between the US President and his counterparts over tariffs.
Larry Kudlow, director of the National Economic Council, lashed out at Trudeau as "amateurish" and "sophomoric" for a news conference the prime minister gave after Trump left the G7 summit on Saturday.
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"It's going to change", said Trump.
The majority of Canadian exports go to the United States, making Canada uniquely vulnerable to a US trade war.
That leaves Trump diametrically opposed with Canada, which says renegotiating the deal every five years would create perpetual uncertainty and harm long-term investment.
Such a move could make it almost impossible to renegotiate the terms of the 1994 NAFTA pact between the United States, Canada and Mexico.
The summit at Quebec's Charlevoix resort failed to produce any truce in an intensifying trade conflict.
"We're like the piggy bank that everybody is robbing", Trump said here at a news conference.
Trump recently ignited a trade dispute by imposing tariffs on his North American and European partners' steel and aluminium exports and repeated a threat to impose tariffs on vehicle imports. "It is in everyone's interest", he said on Twitter according to an English translation.
Trump also defended his recent imposition of punishing steel and aluminum tariffs was based on legitimate national security concerns, and said he stood firm against the concerns of his fellow G7 leaders during their talks.
The president also referred to Trudeau's "false statements", which he didn't explain.
Trump's closest advisers added to the attack during televised interviews on Sunday.