According to a US securities filing, Elon Musk owns 9.2% of Twitter.
Twitter shares soared by more than 27% in New York trading on Monday.
Tesla’s CEO owned 73,486,938 shares of the social media platform as of 14 March, according to the Securities and Exchange Commission.
Based on Twitter’s closing price on Friday, the stake is worth $2.89bn (£2.2bn).
It is more than four times the 2.25% holding of Twitter founder Jack Dorsey, making him the largest shareholder in the company.
Musk has more than 80 million followers on Twitter but recently said he is “seriously thinking” about building a new social media platform.
Musk asked his followers late last month whether the social media platform encouraged free speech.
“Free speech is essential to a functioning democracy. Do you believe Twitter rigorously adheres to this principle?”
He then asked: “Is a new platform needed?”
On March 25, he accompanied it with a poll, with 70 per cent of two million users responding that the platform does not adhere to free speech.
Musk said: ‘The consequences of this poll will be important. Please vote carefully.’
Musk said in a separate tweet that he is considering creating a social media platform.
Musk was responding to a Twitter user’s question about whether he would consider developing a social media platform with an open-source algorithm that prioritized free speech and discouraging propaganda.