After Business Insider inquired about the shares, Williams posted a blog post explaining why he was selling.
"After a year and a half of no selling, I have filed a new 10b5-1 plan to liquidate a minority of my TWTR over the next year," Williams wrote. "This plan kicked in on Monday. It actually pains me to be selling at this point, but this sale is all about personal context, not company context."
Williams sold about $4 million worth of Twitter stock Wednesday, according to SEC filings, but that's just the beginning. He plans to sell up to 30% of his shares over the next few months. The sale would reduce his stake in Twitter to slightly under 4% from about 5%.
Twitter shares fell by more than 1% on the news, after starting Thursday's session trading slightly up.
Though Williams stressed in his blog post that the move did not reflect any loss of confidence in the company he founded 11 years ago, the stock sales by one of the company's high-profile founders is another setback for Twitter as it tries to get back in investors' good graces amid a string of bad news.
Williams will remain on Twitter's board, a representative told Business Insider.
"Evan Williams recently entered into a pre-arranged 10b5-1 stock trading plan as part of his continued efforts to diversify his financial positions and focus on his charitable organization and other endeavors," the person said. "Ev, a co-founder of Twitter and member of the Board since 2007, has been a champion of Twitter since Day 1 and intends to maintain a significant ownership position in the company. The plan is not expected to result in the sale of more than 30% of Ev's holdings."
Williams also served as Twitter's CEO for a brief time. He stepped down in 2010. He's now the CEO of Medium, a blogging platform.
Twitter has gone through a series of struggles over the past few years as its growth has stalled. The company has seen an exodus of top executives, including chief operating officer Adam Bain in November. Jack Dorsey, the cofounder who took over as CEO in 2015, has tried to right the ship even as he serves as CEO for the digital-payments company Square.
But a plan to focus on streaming live video, such as sporting events, has failed to rekindle growth. And on Tuesday news surfaced that Twitter's deal to stream NFL games during the 2016 season was going to the company's rival Amazon for the 2017 season.
Williams said in his blog post that he was optimistic about Twitter's future, and he noted that the "vast majority" of his assets were in Twitter stock. Selling some of his shares would enable him to invest in tech startups, philanthropic efforts, and political donations, he said.
"I'm not a public market investor," Williams wrote. "And I feel very fortunate to be able to use funds to enable other people to do good things."
Williams isn't the only major Twitter investor to dump a lot of shares. Chris Sacca, an early Twitter investor, said in March that he had sold all of his shares since Dorsey took over as CEO.
Here's a look at Twitter's stock reaction to the news:
This post originally appeared on Business Insider.