Tesla’s famed CEO Elon Musk is set to join Twitter’s board of directors just days after taking a passive 9.2% stake that made him the platform’s biggest shareholder. Musk said he would work to drive “significant improvements” to Twitter.

On Monday, Musk revealed that he was the owner of 73.5 million Twitter shares worth roughly $2.9 billion, an announcement that sent the stock up 27%.

The 13G filing, which is used by investors when they do not intend to influence the target company, suggested the investment was passive in nature but on Tuesday the social media platform announced that the billionaire would join its board as part of a letter agreement inked a day before. The two sides were described as holding talks “in recent weeks,” in a tweet by Twitter CEO Parag Agrawal.

Musk will be named a Class II director with a term expiring at Twitter’s 2024 annual meeting, the company said. During his board tenure and 3 months after, Musk will not own more than 14.9% of the company’s share capital.

Musk has repeatedly criticized the social media company in recent years and last week said he was considering building a new social media platform amid concerns Twitter doesn’t allow for free speech.

In a Tuesday tweet welcoming Musk, Twitter CEO Agrawal said the billionaire’s critique of the service “is exactly what we need” to “make us stronger in the long-term.”

On Monday, hours after he made his $2.9 billion investment in Twitter stock public, Musk created a poll on the platform asking his 80 million followers whether they would like to be able to edit their tweets after posting them. Twitter’s chief executive Agrawal quoted Musk’s post and urged people to “vote carefully” as the consequences “will be important.”

“Looking forward to working with Parag & Twitter board to make significant improvements to Twitter in coming months!” Musk said Tuesday.

Shares in Twitter rose as much as 9.2% Tuesday in New York but gave back most of the gains to finish the day at $50.98, up just 2%, giving the social media company a market value of $40 billion.

After the close Tuesday, Musk filed a 13D report, typically used by activist investors, saying he was holding the Twitter stake for “investment purposes.” The report also reveals that the billionaire started buying Twitter shares on January 31.

Musk takes a significant role at Twitter just as co-founder Jack Dorsey is exiting. Dorsey relinquished the CEO role in November and was replaced by Twitter’s chief technology officer Agrawal. Dorsey is set to leave the company’s board at this year’s annual meeting.

Dorsey came under fire from Elliott in 2020 when the activist questioned his ability to run both Twitter and digital payments company Square. Dorsey and Elliott made peace in March 2020 when Twitter struck a settlement with the activist and private equity firm Silver Lake, giving each a board seat and establishing a framework to review the company’s leadership and governance.