In a followup tweet, Trump targeted Trudeau again, saying he "acted so meek and mild during our @G7 meetings only to give a news conference after I left saying that, "US Tariffs were kind of insulting" and he 'will not be pushed around'".
Trump had entered the summit amid a spat with Trudeau and French President Emmanuel Macron over USA tariffs on steel and aluminum.
After departing from the Quebec summit for Singapore, Mr Trump tweeted that he had asked American representatives not to endorse the joint communique put out by the G7 leaders, which called for a "free, fair, and mutually beneficial trade" and the importance of fighting protectionism.
The summit was wrapping up just as Chinese President Xi Jinping began hosting the leaders of Russian Federation and Iran at a regional security meeting in a symbol of the power-play between East and West.
Mr Trump made the comments after the communique had already been officially released in response to a press conference held by Justin Trudeau to mark the close of the summit.
Trump's bombshell announcement that he was backing out of the Group of Seven communique, made after he left the summit in Canada early, torpedoed what appeared to be a fragile consensus on the trade dispute between Washington and its top allies.
"No tariffs, no barriers, that's the way it should be, and no subsidies", Trump said on Saturday during a 30-minute press conference in La Malbaie. "Or we're going to make a deal directly with Canada, directly with Mexico", Trump said Saturday.
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Trump had come to Quebec insisting on his long-standing claim that America has been exploited for too long by existing trade arrangements - and he was met by counterparts equally determined to protect the "rules-based" global system.
Trudeau's view of how the summit went was a marked contrast to Trump's take.
European officials at the summit were shocked by the idea, or dismissive, and Tusk, while admitting that it was hard to be surprised by Trump's pronouncements any more, said: "Let's leave the G7 as it is".
Such a move could make it almost impossible to renegotiate the terms of the 1994 NAFTA pact between the United States, Canada and Mexico. "Our tariffs are in response to his of 270 per cent on dairy!"
"The EU understands that the only way with Trump is strength", said one European official.
He also defended the wisdom of his controversial tariffs, saying that the USA would win a trade war against its allies because "they do so much more business with us than we do with them".
"I highlighted that it was not helping in our renegotiation of NAFTA and that it would be with regret, but it would be with absolute certainty and firmness that we move forward with retaliatory measures on July 1".
He said US farmers had been harmed by tariffs and other barriers and warned that USA trading partners would need to provide him with more favorable terms.