Stone posted his return to the social media company in a Medium post on Tuesday, writing that he'll be "filling the 'Biz-shaped hole'" he left there.
"My top focus will be to guide the company culture, that energy, that feeling," Stone wrote, adding: "You might even say the job description includes being Biz Stone."
Jelly Industries, "a human-powered search and discovery engine" and Stone's latest start-up, was acquired by Pinterest in March. Stone was brought on as a "special advisor" for Pinterest as part of the deal.
Chief operating officer Adam Bain was last year, leaving Anthony Noto in charge of the company's finances and operations. CEO Jack Dorsey is also running , a job top shareholders say . And the company has yet to formally announce a replacement for former chief technology officer Adam Messinger or product gurus Noah Pepper and Jinen Kamdar.
While Twitter's influence has grown alongside that of active user Donald Trump, it has failed to make a large dent in the revenues of dominant advertising platforms like Facebook and Google. Users, meanwhile, have bemoaned the company's slow response to issues like harassment and the lack of an "edit" function.
Stone will also meet again with fellow co-founder Ev Williams. Williams and Dorsey have a history of clashing over the CEO post— which has reportedly created turmoil in Twitter's top ranks. Stone, on the other hand, has listed Dorsey as his "closest friend at the company," and worked closely to help found Medium with Williams.
"It's important that everyone understands the whole story of Twitter and each of our roles in that story," Stone wrote. "I'll shape the experience internally so it's also felt outside the company."