The US Securities and Exchange Commission said Musk's failure to disclose the price led to him paying $150m in shares.
Elon Musk is being sued by the United States securities regulator because he did not disclose his share of Twitter in time before buying the social network.
The US Securities and Exchange Commission (SEC) said on Tuesday that Musk failed to disclose within 10 days that he acquired more than 5% of Twitter's revenue in March 2022.
The failure of the CEO of Tesla and SpaceX to inform the management allowed him to continue to buy shares at “increasingly low prices”, the SEC said in a filing with the US District Court for the District of Columbia.
Musk's actions allowed him to “pay at least $150 million for the shares he bought after the beneficial ownership report,” the SEC said.
Musk finally notified regulators that he had acquired more than 9 percent of Twitter's assets on April 4, 2022, 11 days after the disclosure, the SEC said.
Twitter's stock price that day rose 27 percent from the previous day's close, according to the regulator.
“Because Musk did not disclose his beneficial ownership in time, he was able to buy this from unsuspecting people at very low prices, which did not yet reveal the undisclosed amount of Musk's beneficial ownership of more than 5 percent of Twitter's common stock,” said the security manager.
“In total, Musk paid less than his Twitter stock and more than $150 million for buying Twitter's common stock during this period. Investors who sold Twitter's common stock during this period did so at low prices and thus suffered significant financial losses.”
US securities laws require investors who buy more than 5 percent of a company's shares to disclose their holdings so they can make informed investment decisions.
The SEC has sued Musk twice before, including on Twitter in 2018 when he said he raised money to take electric car company Tesla private.
Musk settled the lawsuit by paying a $20m fine, agreeing to a legal review of some of his televised actions, and stepping down as chairman of Tesla.
Musk ended up buying Twitter in October 2022 for $44bn, after signing a purchase agreement that he later tried to renege on.
The SEC's latest action, announced with little fanfare, comes days after Chairman Gary Gensler is expected to step down on January 20, the day of President Donald Trump's inauguration.
It is unclear whether the lawsuit against Musk, one of Trump's closest allies, will continue under Trump, who named Paul Atkins, a former director of the SEC, as Gensler's successor.