"It's been an incredible 13 years, and I'm proud of what Twitter has accomplished during my time with the company," Williams said in a statement. "I will continue rooting for the team as I focus my time on other projects."
He took over the CEO role from founder Jack Dorsey in 2008 before ceding that role to outsider Dick Costolo in 2010. He left the company in 2011, and Dorsey eventually reclaimed the CEO role in 2015.
Over the past few months, Williams has been drastically decreasing his stake in Twitter. He owned about 14.6 million shares of the company as of December, down from 29 million shares last April.
Williams is currently the CEO of Medium, a San Francisco tech company that allows writers to publish long-form pieces. Medium recently achieved notoriety as the venue where Jeff Bezos posted his response to alleged threats from the National Enquirer to publish explicit photos of him.