Trudeau also referenced the recent plant closures by General Motors that will affect almost 15,000 jobs in the United States and Canada, calling the development a "heavy blow".
Trump ceremoniously signed the new agreement, called the U.S. Mexico Canada Agreement, in Buenos Aires this morning with Canada Prime Minister Justin Trudeau and Mexico's President Enrique Peña Nieto.
ME retailers that ship products to Canada could see a sales boost under a new North American free trade deal.
The vast majority of the pact still needs to be ratified by lawmakers in the three countries but the signing enacts a handful of immediate protections, such as from auto tariffs.
He also said, "the task isn't done, more hard work is ahead".
USA objections to Canada's protected internal market for dairy products was a major challenge facing negotiators during the talks, and Trump repeatedly demanded concessions and accused Canada of hurting U.S. farmers.
"Battles sometimes make great friendships", Trump said, looking to put a positive gloss on his fraught relationship with Canadian Prime Minister Justin Trudeau.
Stop It, Tyson Fury Did Not Earn a Win Against Deontay Wilder
Wilder agreed with Fury, explaining that "I 100% agree with that". Me and this man are the two best heavyweights on the planet". That makes him slightly shorter than Fury, who is 6ft 9in.
The United States-Mexico-Canada Agreement (USMCA) opens 3.6 per cent of Canada's dairy market to tariff-free imports and limits Canada's global dairy exports of skim-milk powder, baby formula and milk-protein concentrates.
Canada's dairy lobby urged Trudeau this week not to sign the agreement unless it was changed to give the US less influence over the industry. He said that NAFTA had a bad connotation in the US due to the negative effects of the agreement in the country, and that it is better to start fresh. Before Trump arrived in Argentina he injected additional drama into the proceedings by cancelling another high-stakes meeting, with Russian President Vladimir Putin.
BDI, Germany's main industry association, said in a statement that the autos rules of origin were "a retrograde step compared with NAFTA".
Pena Nieto, who will hand off to his successor Saturday, said he was honoured to be at the signing on the final day of his administration, calling it the culmination of a long process "that allow us to overcome differences and to conciliate our visions".
"These new provisions will benefit labor, technology and development in each of our nations", Trump said Friday.
Trump, standing in between his Canadian and Mexican counterparts, said the new deal was a model agreement that would stop auto jobs from going overseas, protect intellectual property rights and provide robust protections for digital trade and financial services. He struck a more optimistic note on Friday, saying, "It's been so well-reviewed, I don't expect to have very much of a problem". However, ongoing differences over steel and aluminium tariffs the US has imposed on its northern and southern neighbours encroached on the celebratory moment.
Shortly after the signing, Mr Trump tweeted: "The awful NAFTA will soon be gone".
"Make no mistake, we will stand up for our workers and fight for their families and their communities", he said.
Trump was greeted warmly at the Casa Rosada by Argentine President Mauricio Macri, a longtime business acquaintance.