The results clinch a crucial vote of confidence that strengthens Milei's ability to carry out his radical free-market experiment with billions of dollars in backing from the Trump administration.
Milei's governing La Libertad Avanza party won more than 40 per cent of votes in national elections to renew almost half of the lower house of Congress, according to tallies in local media using numbers from electoral authorities with more than 97 per cent of votes counted.
La Libertad Avanza also swept six of the eight provinces in the vote to renew a third of the Senate. The figures exceeded analysts' projections for Sunday's elections.
In comparison, the results showed the left-leaning populist opposition movement, known as Peronism, winning more than 31 per cent of the vote, in what analysts described as the alliance's poorest performance in years.
Milei said his party went from holding just 37 seats in the lower house of Congress to 101 after Sunday's vote.
In the Senate, he said La Libertad Avanza picked up 14 more seats to end up with 20 senators.
The strong showing ensures Milei will have enough support in Congress to uphold presidential vetoes, prevent an impeachment effort and see through his ambitious plans for tax and labour reforms in the coming months.
At his party headquarters in downtown Buenos Aires, Milei burst onstage and sang a few lines of the death-metal tune that has become his anthem in a raspy baritone: "I am the king of a lost world!"
Beaming as his supporters cheered, he seized on the results as evidence that Argentina had turned the page on decades of Peronism that brought the country infamy for repeatedly defaulting on its sovereign debt.
"The Argentine people left decadence behind and opted for progress," Milei said, thanking "all those who supported the ideas of freedom to make Argentina great again".
Before the election, US President Donald Trump indicated he could rescind $US20 billion ($A31 billion) in financial assistance to his close ally if Milei lost the vote.
Even though voting is compulsory, electoral authorities reported a turnout rate of just under 68 per cent, among the lowest recorded since the nation's 1983 return to democracy.
Milei, a key ideological ally of Trump who has slashed state spending and liberalised Argentina's economy after decades of budget deficits and protectionism, had a lot riding on Sunday's elections.
Milei's government has been scrambling to avert a currency crisis ever since a major defeat by the Peronist opposition in a provincial election in September panicked markets and prompted a sell-off in the peso that led to the US Treasury's extraordinary intervention.
A series of scandals — including bribery allegations against Milei's powerful sister, Karina Milei — hurt the president's image as an anti-corruption crusader and hit a nerve among voters reeling from his harsh austerity measures.
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