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.
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?”
Given that Twitter serves as the de facto public town square, failing to adhere to free speech principles fundamentally undermines democracy.
What should be done? https://t.co/aPS9ycji37
— Elon Musk (@elonmusk) March 26, 2022