The final version of the USMCA "is infinitely better than what was initially proposed by the administration," Pelosi said.
The USMCA replaces the North American Free Trade Agreement (NAFTA), which took effect on January 1, 1994.
The renegotiation of NAFTA began in 2017 with the arrival in the White House of Donald Trump, who made overhauling the trade agreement a priority, Efe news reported.
During the 2016 presidential campaign, Trump called NAFTA "one of the worst trade deals in history" and blamed the Clinton-era agreement for the loss of tens of thousands of manufacturing jobs in the United States.
On Tuesday, the US president pushed for Pelosi and her fellow lawmakers in the Democratic-controlled House to back the USMCA.
"America's great USMCA Trade Bill is looking good. It will be the best and most important trade deal ever made by the USA. Good for everybody - Farmers, Manufacturers, Energy, Unions - tremendous support. Importantly, we will finally end our Country's worst Trade Deal, NAFTA!" Trump said on Twitter.
The White House and the House Democratic leadership had held talks for months in an effort to win approval of the USMCA.
"This is a day we've all been working to and working for on the path to 'Yes,'" Pelosi said.
Mexican President Andres Manuel Lopez Obrador, for his part, said Tuesday that he, US Trade Representative Robert Lighthizer and Canadian Deputy Prime Minister Chrystia Freeland would sign the USMCA during a ceremony in Mexico City.
While the overhaul of NAFTA was spearheaded by President Enrique Peña Nieto, who governed Mexico from 2012 to 2018, Lopez Obrador's team had observer status during the negotiations, which dragged on for more than one year.
After the new version of the USMCA is signed, lawmakers in the three countries are expected to ratify the trade agreement.
The House of Representatives is expected to vote on the trade agreement next week, approving it before the congressional recess starts on December 20, media reports said.
The USMCA received the backing of organized labor, a key supporter of the Democratic Party.
"Make no mistake, we demanded a trade deal that benefits workers and fought every single day to negotiate that deal; and now we have secured an agreement that working people can proudly support," AFL-CIO President Richard Trumka said in a statement.
A key change in the USMCA is a requirement that 75 per cent of cars be made in North America, up from 62.5 per cent under NAFTA. Additionally, between 40 per cent and 45 per cent of a vehicle's content must be produced by workers who earn at least $16 per hour, a stiffer requirement that could lead to more cars being manufactured in the US and Canada, as opposed to low-wage Mexico.
The deal also offers US dairy farmers greater access to the Canadian market and includes new provisions regarding e-commerce and intellectual property rights.
( With inputs from IANS )