"I'm happy to announce that we've released a joint communique by all seven countries", Trudeau said.
But its validity was immediately put into question when Trump tweeted that he had "instructed our USA reps not to endorse the communiqué".
G7 leaders had meant to use the summit to confront Trump on the tariffs, which they have described as protectionist and a threat to the global economy.
When a CNN reporter asked Trump to respond to a question about whether he had sowed division in the G7 in what many have described as a G6-plus-one scenario, the president excoriated the reporter for perpetuating "fake news".
He said the U.S. required fair access to markets and an end to unfair trade practices, telling reporters the "United States has been taken advantage of for decades and decades".
The move was in response to the USA tariffs, which include a 25 per cent tax on steel and a 10 per cent tax on aluminum imports from Canada and many other allied countries.
Trudeau told Trump that it was "unacceptable" to cite national security when targeting a military ally like Canada.
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Trump came out swinging in his news conference, repeating his view that America has always been cheated by its friends.
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This offended many Indians and Priyanka Chopra received a lot of flak for the same. "I sincerely apologise. I'm a proud Indian and that will never change".
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Meanwhile, the European Union - the world's largest trading bloc - announced last week it would impose retaliatory tariffs on the USA beginning in July, over Washington's duties on steel and aluminum imports from Europe.
The government announced it would impose more than $16.6-billion in retaliatory tariffs, effective July 1, on a variety of US goods. "I think within the first minute I'll know", Trump said.
And he said he told Trump "it would be with regret but it would be with absolute clarity and firmness that we move forward with retaliatory measures on July 1, applying equivalent tariffs to the ones that the Americans have unjustly applied to us".
The communique said the leaders of the United States, Canada, Britain, France, Italy, Germany, and Japan agreed on the need for "free, fair, and mutually beneficial trade" and the importance of fighting protectionism.
"We are focused on everything we accomplished here at the summit".
Trump's tirade came before he was to leave for Singapore for his historic summit with the North Korean leader North Korea's Kim Jong Un.
Although Trump says his tariffs are necessary to protect United States industry and workers, Canada and the European Union have denounced them as illegal.
The Prime Minister said nothing he hasn't said before - both in public, and in private conversations with the President.
British Prime Minister Theresa May said the other G7 leaders registered their "deep disappointment at the unjustified decision by the U.S." on its closest allies. "I think it would be good for the world, I think it would be good for Russian Federation, I think it would be good for the United States, I think it would be good for all of the countries of the current G7", Trump said.
Trudeau said the summit gave leaders an "opportunity to join forces, and sometimes to settle disputes".