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 the largest shareholder 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 because he believed Twitter was failing to adhere to free speech principles.
His own use of the platform has got him into trouble in the past.
Musk is locked in a legal battle to free himself from the constraints of a deal he made with US regulators in 2018. The agreement forces him to have his tweets about Tesla pre-approved by a lawyer to avoid breaking rules around the disclosure of market sensitive information.
Musk signed an agreement with the US Securities and Exchange Commission (SEC) after he claimed, in a tweet, that he had secured funding to take the company private at $420 (£325) a share.