A day after asking his followers on his X platform whether a new US political party should be created, Musk declared in a post on Saturday that "Today, the America Party is formed to give you back your freedom."
"By a factor of 2 to 1, you want a new political party and you shall have it!" he wrote.
The announcement from Musk comes after Republican President Trump signed his self-styled "big, beautiful" tax-cut and spending bill into law on Friday, which Musk fiercely opposed.
Musk, who became the word's richest man thanks to his Tesla car company and his SpaceX satellite firm, spent hundreds of millions on Trump's re-election and led the Department of Government Efficiency from the start of the president's second term aimed at slashing government spending.
The first sign of investor dissatisfaction with Musk's announcement followed later in the day.
Investment firm Azoria Partners will postpone the listing of a Tesla exchange-traded fund, Azoria CEO James Fishback said in a post on X.
Fishback, who has asked Tesla's board to clarify Musk's political ambitions, said the new party undermined the confidence shareholders had that he would focus more on the company after leaving government service in May.
Musk said previously that he would start a new political party and spend money to unseat lawmakers who supported the bill.
Trump earlier this week threatened to cut off the billions of dollars in subsidies that Musk's companies receive from the federal government.
Republicans have expressed concern that Musk's on-again, off-again feud with Trump could hurt their chances to protect their majority in the 2026 midterm congressional elections.
Asked on X what was the one thing that made him go from loving Trump to attacking him, Musk said: "Increasing the deficit from an already insane $US2T under Biden to $2.5T. This will bankrupt the country."
There was no immediate comment from Trump or the White House on Musk's announcement.
The feud with Trump, often described as one between the world's richest man and the world's most powerful, has led to several precipitous falls in Tesla's share price.
The stock soared after Trump's November re-election and hit a high of more than $US488 ($A745) in December, before losing more than half of its value in April and closing at $315.35 on Friday.
Despite Musk's deep pockets, breaking the Republican-Democratic duopoly will be a tall order, given that it has dominated American political life for more than 160 years and Trump's approval ratings in polls during his second term have generally held firm above 40 per cent, despite often divisive policies.
