President Trump’s breakthrough in trade talks with Chinese President Xi Jinping got cheers Sunday from top business executives who had criticized the administration’s get-tough tariff tactics.

The Business Roundtable, a group of chief executive officers from America’s major corporations, said it was “very encouraged” by the outcome of the talks Saturday in Buenos Aires.

“We are hopeful this will lead to both important reforms in China and a de-escalation in trade tensions between the U.S. and China,” the group said in a statement.

The breakthrough occurred at a more than two hour dinner meeting in Buenos Aires, where Mr. Trump and Mr. Xi were attending a summit for the Group of 20 leaders.

The U.S. agreed to a 90-day delay of higher tariffs that were set to hit China on Jan. 1, and Beijing agreed to begin opening its markets to American goods.

The details of China’s market reforms still need to be ironed out before the new tariff deadline in April. But the talks appeared to advance toward Mr. Trump’s goal without major concessions by the U.S.

“It’s an incredible deal,” Mr. Trump told reporters on the return trip aboard Air Force One. “If it happens, it goes down as one of the largest deals ever made.”

He said they managed to make progress despite Friday’s death of former President George H.W. Bush overshadowing the meetings.

Mr. Trump has weathered fierce criticism over tariffs from free-trade advocates, including the Business Roundtable and Capitol Hill Republicans.

While the roundtable pushed for eased tensions on all trading fronts, it acknowledged the importance of ending China’s unfair trade practices that has been the cornerstone of Mr. Trump’s trade agenda.

“China’s trade and investment practices have prevented U.S. companies from competing on a level playing field and hurt U.S. workers. The Administration is right to demand reforms that liberalize the Chinese market and ensure fair treatment of U.S. companies,” said the group.

Mr. Trump has used tariffs and other get-tough measures to force trade negotiations on several fronts, including prodding the European Union to the bargaining table and reworking the 24-year-old North American Free Trade Agreement.

While in Argentina, Mr. Trump and leaders of Mexico and Canada signed the new U.S., Mexico Canada Agreement, or USMCA, to replace NAFTA.

He later announced that he would proceed with terminating NAFTA to put a six-month deadline on Congress to approve USMCA.

The new three-way pact likely faces a tough road through the new Congress in which Democrats will control the House.

“So Congress will have a choice of the USMCA or pre-NAFTA, which worked very well,” he told reporters. “You got out, you negotiate your deals. It worked very well.”

Rep. Bill Pascrell Jr. of New Jersey, the top Democrat on a House trade subcommittee, said he wasn’t impressed by the president’s moves on China or NAFTA.

“I’ll be watching whether this truce actually delivers or if Mr. Trump will just blow things up again,” he said.

With China, Mr. Trump is demanding an end to longtime abuses such as theft of intellectual property, subsidies for Chinese companies, and the use of high duties and other trade barriers against American goods.

Trade talks had stalled as of late, but China, with a flagging economy, badly wanted to avoid the next round of tariffs.

Tariffs on $200 billion of Chinese goods would have risen from 10 percent to 25 percent on Jan. 1. Mr. Trump also threatened tariffs on another $267 billion of imports, which would effectively tax all goods from China.

At the dinner, Beijing agreed to buy a substantial amount to U.S. agriculture, energy and industrial goods, although the amount have not been finalized, according to the White House.

At the two-day G20 summit, Mr. Trump also marshaled international support for overhauling the World Trade Organization, which goes hand in hand with his confrontation of China’s trade abuses.

The call for WTO reforms was part of the joint statement issued by the G20 leaders at the conclusion of the summit Saturday.

The non-binding agreement, negotiated in all-night talks, papered over deep division between member nations, including Mr. Trump’s lone opposition to the Paris climate accord.

In the statement, China also agreed to more transparency in financing of infrastructure projects around the world that has stoked fears of Beijing taking over roads, railways and ports in developing countries.

Mr. Trump’s trip to Argentina got off to an inauspicious start with his abrupt decision to call off a planned meeting with Russian President Vladimir Putin.

Mr. Trump cited the Russian coast guard last week firing on and seizing three Ukraine navy vessels off the coast of Crimea. Moscow and the president’s critics in the U.S. suggested the real reason could have been revelations that special counsel Robert Mueller’s probe was looking into Mr. Trump’s business dealings in Russia.

Mr. Trump had an “informal conversation” with Mr. Putin at a dinner Friday with G20 leaders, the White House said without providing details.