The decisive victory of President Javier Milei of Argentina in legislative elections on Sunday gave him a critical boost, with voters from the mountains of Patagonia to the farmlands in the Pampa casting their ballot to back his sweeping economic experiment.

Some 8,000 miles north of Buenos Aires, in Washington, the Trump administration was also counting it as a victory.

“These results are a clear example that the Trump Administration policy of peace through economic strength is working,” Treasury Secretary Scott Bessent, who weeks ago announced a $20 billion lifeline for Argentina, wrote on X on Monday.

“He had a lot of help from us,” President Trump said of Mr. Milei as he traveled in Asia on Monday for meetings with other world leaders. “We’re getting a real strong handle on South America.”

Financial markets reacted euphorically to the results as Mr. Milei emerged from the election with an even tighter bond to the United States and a firmer mandate to pursue more transformative changes to the country’s economy.

The resounding victory was unexpected for Mr. Milei, a libertarian economist who had been facing significant political and financial turmoil in recent weeks. Many voters had grown disenchanted with austerity measures and corruption scandals embroiling Mr. Milei’s inner circle.