He said the US paid close to the entire cost of Nato to help protect countries that rip us off on trade.
He added that fair trade is now to be called fool trade, in response to the threat of new tariffs against the US.
His tirade came after G7 allies criticised his decision to reject a joint communique.
German Chancellor Angela Merkel said on TV that his change of heart was "sobering and a little depressing".
The French presidency had earlier condemned "fits of anger".
After Trump had left the summit in Quebec, he rounded on Justin Trudeau personally, suggesting the Canadian prime minister was "very dishonest and weak" and "acts hurt when called out".
In a news conference Trudeau had reasserted his opposition to US tariffs on steel and aluminium, and vowed to press ahead with retaliatory moves on 1 July.