Jack Dorsey, Twitter's product mastermind and the last remaining co-founder still working at the social media site, has stepped back from day-to-day operations to focus on his other start-up, payments firm Square.
The confirmation of his role on Tuesday came as Twitter makes changes to how it works with external partners and rapidly scaling up its advertising business.
One of Silicon Valley's best-known product designers, Mr Dorsey conceived the original idea for Twitter and became its first chief executive in 2006 until he was swept aside by co-founder Evan Williams in 2008. In March 2011, Mr Dorsey was brought back to Twitter as executive chairman by its current chief executive, Dick Costolo, to oversee its product, design and branding.
Last November, Mr Dorsey said he was working full-time at both Twitter and Square every day, managing the rapid growth of the mobile payments service while smoothing some of Twitter's rougher edges from its service and branding.
In a posting to his personal website, Mr Dorsey said that he was now spending the "majority" of his time at Square, returning to Twitter only one afternoon a week, having quietly scaled back his duties earlier in the year.
"We haven't talked about this publicly because it's not what people using Twitter every day care about," Mr Dorsey said. "I'm fortunate in life to be a part of two foundational and mission-driven organisations, and I'm always going to fight like hell to make them thrive."
Last weekend the New York Times reported that other Twitter employees had found Mr Dorsey "difficult to work with" and indecisive.
In his statement on Tuesday morning, Mr Dorsey admitted that his role at Twitter was "not a common arrangement" but said that it had always been his and Mr Costolo's "shared goal" to move his operational responsibilities back to the chief executive.
Mr Costolo tweeted in response that he was "fortunate to be working with a clear thinker like @jack".
In the past year, Twitter has overhauled its desktop site and mobile apps, as well as redesigning its logo and creating new events pages for partners, such as broadcaster NBC during the London Olympic Games and Nascar motor racing.
But it remains in the midst of controversial changes to its developer platform, which has caused consternation both inside and outside the company.
Twitter's other co-founders, Mr Williams and Biz Stone, left last year, shortly after Mr Dorsey's return, to found a new start-up incubator, called Obvious. Although Mr Dorsey fell out with his co-founders, he is said to remain on good terms with Mr Costolo.
On Monday, Twitter announced that it had hired a new vice-president of design, Mike Davidson, formerly of NBC News, who said he hoped to make the rapid-fire messaging service "smarter and more responsive to what people want from it".