He said that it "lifts the risk of serious economic uncertainty that lingers throughout a trade renegotiation process - uncertainty that would have only gotten worse and more damaging if we had not reached a new NAFTA".
"We have worked hard on this agreement", Trump said.
Nieto explained the USMCA "is the first trade agreement that incorporates elements that address the social impact of global trade; it enables the participation of more sectors in the economy".
Trudeau made a personal plea to Trump during the ceremony to get rid of the tariffs, saying the recent closures of auto plants by General Motors in the United States and Canada made it more important.
When the three countries reached the deal at the end of September, Trump said he was not at all confident of obtaining congressional approval.
Asked Thursday why the two had no meeting scheduled, Trump said: "I would have met with him but we didn't set that one up". The awful NAFTA will soon be gone. 'The fact that this is an agreement in three languages adds to the level of technical complexity and it is on that level that we are just being sure that all the Is are dotted and all the Ts are crossed'.
On Saturday, he will meet with Chinese counterpart Xi Jinping to address the trade war triggered by Trump's insistence on China ending what he says have been decades of unfair trade practices.
Meanwhile, Mr Trump's other major trade-related meeting during the G20 summit will be with China's leader Xi Jinping on Saturday, with analysts looking for any sign that the United States president will hold off on plans to toughen U.S. tariffs on Chinese imports in January.
Among other things, the agreement also lengthens copyrights for creative works and patent protection for certain sophisticated medicines, slightly opens up the protected Canadian dairy industry to USA access and eliminates the long-controversial "Chapter 11" provision between Canada and the US allowing investors to sue governments for allegedly violating their rights.
The U.S. -Mexico-Canada Agreement is meant to replace the 24-year-old North American Free Trade Agreement, which Trump has long denigrated as a "disaster".
Mr Trudeau told Trump at the signing that the two leaders should work together to reduce those tariffs. On Friday, the president described the prime minister as a friend.
Trudeau and Trump discussed their plans for the G20 summit in a November 27 phone call, according to the White House.
Also inking the USMCA were Canadian Prime Minister Justin Trudeau and Mexican President Enrique Peña Nieto, who signed it on his last day in office.
Trudeau's readout on the same call also mentioned lifting the tariffs the USA imposed on steel and aluminum.