The billionaire bought 9.2 per cent of the company, which is worth almost $3bn (£2.3bn), according to a filing with the US Securities and Exchange Commission.
“Oh hi lol,” has been his only tweet since the announcement, which sent shares in Twitter up more than 25 per cent.
Mr Musk is one of Twitter’s most popular users, with more than 80 million followers. He is also particularly prolific, using his account to share memes as well as update on his life and his companies.
Those tweets have however caused problems for Mr Musk in the past. In 2018, for instance, he tweeted that Tesla would go private – starting a period that saw him investigated by the SEC as well as the loss of his position as chair of the company.
In recent weeks, Mr Musk has used his account to post about Twitter itself, and the rules that govern it.
On 24 March, he tweeted about being “worried about de facto bias in ‘the Twitter algorithm’ having a major effect on public discourse”, and shared a poll inviting his followers to vote on whether that algorithm should be open source.
A day later, he tweeted that “free speech is essential to a functioning democracy”, and polled his followers on whether they thought “Twitter rigorously adheres to this principle”. After 70.4 per cent of respondents voted that it did not, he asked followers “what should be done?”
Mr Musk bought his Twitter shares before those posts – filings with the SEC indicate the purchase happened on 14 March – though they were only disclosed today.