The Tesla chief took a 9.2% stake in the social media site on March 14, according to a Securities and Exchange Commission filing.
The stock purchase makes Musk the social network's biggest shareholder, Bloomberg reported Monday.
The shares are worth $2.89 billion, based on Twitter's closing price Friday, according to CNBC. Twitter's stock soared more than 27% on Monday, closing at $49.97.
Musk is among Twitter's most prolific users and has used his reach to question the platform's commitment to free expression.
On March 25, he polled his 80.2 million followers, asking if they thought Twitter "rigorously adheres" to the principle of free speech. More than 70% said no.
Musk was referring to free speech protections under the First Amendment to the US Constitution, though that only applies to government censorship of speech.
Twitter, as a company, determines what can be posted on its platforms, requires users to agree to terms of service when they sign up and can remove people who break its rules.
Twitter has been undergoing major leadership changes. In November, Twitter co-founder Jack Dorsey stepped down as the company's chief executive.
Parag Agrawal, who was previously Twitter's chief technology officer, took over the executive role.
Twitter reported $5 billion in annual revenue in 2021 and currently has 217 million daily users who currently see ads. Twitter has a goal of reaching at least $7.5 billion in annual revenue and 315 million users in 2023.