USMCA is a new version of the old NAFTA deal that Trump has sold as one of the biggest achievements of his presidency, providing better conditions for US workers.
"This is a model agreement that changes the trade landscape forever," Trump said at the signing ceremony in Buenos Aires, on the sidelines of the summit.
He said although the parliaments of each country will need to ratify it, there should be no problem getting it through the US Congress.
The build-up to these two days of talks has been dominated by Trump's tough stance on trade and climate, as well as by simmering tensions with Russia.
While the outcome of Saturday's talks between Trump and Chinese President Xi Jinping hangs in the balance, there are hopes the heads of the world's top two economies can find a way to ease the trade row that has seen them impose steep import tariffs.
European stock values were marginally down in early trading on Friday as investors nervously await the outcome of the China-US talks. Asian share prices remained upbeat.
Climate change is likely to be another major sticking point at the summit.
French President Emmanuel Macron has said he will refuse to sign a trade deal with South America's Mercosur bloc if Brazilian President-elect Jair Bolsonaro withdraws from the COP21 Paris climate accord.
The Buenos Aires summit is also being seen as a diplomatic test for Saudi Crown Prince Mohammed bin Salman because of continuing questions about the Saudi state's possible involvement in the murder of journalist Jamal Khashoggi in Turkey last month.