Elon Musk has taken a 9.2% stake in Twitter, according to a US securities filing.
The news sent Twitter shares soaring about 25% in pre-market trading.
The Tesla founder bought 73,486,938 Twitter shares on 14 March, according to the Securities and Exchange Commission.
The stake is worth $2.89bn (£2.20bn), based on Twitter’s closing price on Friday.
The stake makes him one of the largest shareholders in the company and is more than four times the 2.25% holding of Twitter founder Jack Dorsey.
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 Musk asked his followers whether they thought the social medial 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?”