Elon Musk has taken a 9.2% stake in Twitter, according to a US securities filing.
The announcement sent Twitter shares soaring by more than 27% in New York trading on Monday.
Tesla's chief executive owned 73,486,938 shares in the social media platform as of 14 March, according to the Securities and Exchange Commission.
The stake is worth $2.89bn (£2.2bn), based on Twitter's closing price on Friday.
It makes him the largest shareholder in the company, with more than four times the 2.25% holding of Twitter co-founder Jack Dorsey.
Mr Musk is a regular Twitter user with more than 80 million followers, although recently he said he is giving "serious thought" to building a new social media platform.
Late last month, Mr Musk asked his followers whether they thought 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?"
Given that Twitter serves as the de facto public town square, failing to adhere to free speech principles fundamentally undermines democracy.— Elon Musk (@elonmusk) March 26, 2022
What should be done? https://t.co/aPS9ycji37
He regularly uses Twitter to share updates from the companies he owns - including SpaceX and Neuralink. He is also known for sharing memes, adding to his popularity among fans.
But some posts have drawn controversy.
Last year he tweeted in response to a claim, made by the head of the UN World Food Programme (WFP), that just 2% of Mr Musk's wealth could help to solve world hunger.
In October, Mr Musk said he would sell $6bn in Tesla stock and donate it to the WFP, provided it could describe "exactly how $6bn will solve world hunger".
Mr Musk saw the valuation of his Tesla car company surpass a market value of $1 trillion last autumn, making it the fifth such firm to reach the milestone, after Apple, Microsoft, Amazon and Google-owner Alphabet.
Soon after he took to Twitter to ask users if he should sell a 10% stake in the electric carmaker.
More than 3.5 million Twitter users voted, with nearly 58% voting in favour of the share sale leading to Musk selling around $5bn (£3.7bn) of shares in the firm in November.