"It really is an incredible way to end a presidency", Trump said.
The new deal is entitled the US, Mexico and Canada Agreement (USMCA).
More information about Nebraska's principles for trade negotiations with Mexico and Canada can be found by clicking here.
Trump hailed the deal as the biggest and most balanced trade agreement in history, adding the three partners would all benefit.
It will also receive exemptions from any future USA tariffs on 2.6 million imported passenger vehicles. But Ottawa, Mexico City and Washington have now agreed to sweeping changes to manufacturing and labor requirements that US officials say should boost wages and discourage moving production offshore.
The United States-Mexico-Canada Agreement (USMCA) was signed as a replacement to NAFTA on Friday at the G20 summit in Buenos Aires, Argentina, but there still seems to be some confusion over what to call it.
Despite Trump's hostile rhetoric about unauthorized immigrants crossing into the United States from Mexico, he appeared to be on better terms with that nation's president, Enrique Pena Nieto, who was serving his final day in office - and, in Trump's view, going out on a high note by signing the trade agreement. Trump acknowledged the often hard path during negotiations and praised the outcome.
"Make no mistake, we will stand up for our workers and fight for their families and their communities", he said.
The USMCA covers such areas as tariffs, automobile industry, the intellectual property rights, labour standards, protection of the environment and agriculture and would replace the North American Free Trade Agreement (NAFTA).
Already there are calls for changes from the U.S. Congress, while Lopez Obrador's Morena party, which holds a majority in Mexico's Senate, may also seek revisions.More news: Catastrophic Northern California fire is finally contained
The chapter on agriculture in the deal has reportedly not alleviated the Canadian agriculture industry's fear that the United States strong-armed its way to getting more than Canada meant to give up, according to a Canadian newspaper report.
Trump expressed optimism about getting the deal blessed by US lawmakers. "I look forward to a meaningful summit again as soon as this situation is resolved!"
The new agreement is now expected to face staunch Congressional opposition as newly empowered House Democrats will be reluctant to hand the president a political win absent significant concessions in other policy disputes.
With the signing, "the whole region becomes a good basis to invest in the auto industry", Jesus Seade, the NAFTA negotiator for Lopez Obrador, said in an interview with Bloomberg Television on Thursday night.
The new NAFTA deal was drafted on September 30, after a month of rocky negotiations. There will have to be more investment by Toyota, Honda, Mercedes, and the Germans and Koreans. It covers the same three countries as before.
And at 25 years old, NAFTA was in dire need of an update, especially when it comes to areas that didn't exist a quarter-century ago, such as ecommerce, said Matthew Shay, president and CEO of Washington, D.C. -based NRF, the world's largest retail trade association, whose members include grocers.
Meanwhile, a number of lawmakers and business groups are trying to push for the removal of USA tariffs on steel and aluminum imports from Canada and Mexico, which have inflicted harm upon American manufacturers and consumers, raising costs for a wide array of industries.
"The Section 232 tariffs on steel and aluminum still cast a shadow over the zero-tariff philosophy of the USMCA", ACC said.
Trump said he expects the new deal to stop jobs from moving overseas and bring back jobs lost under the original NAFTA.
Trudeau pressed for a resolution to the tariff spat.
"The agreement that we have signed today will allow each country to gain individually, but also North America will grow stronger and will grow to be more prosperous", he said. Trump gave no indication of his plans on the subject.