"Twitter co-founder Jack Dorsey, who was pushed out of the company as chief executive in 2008, is to return full time to lead product development.
He will replace fellow co-founder Evan Williams, who has stepped back from daily involvement with Twitter but will remain on the board.
The announcement was tweeted by chief executive Dick Costolo who wrote that he was "excited" by the return.
Mr Dorsey followed up the tweet, saying that he was "thrilled".
He says he intends to remain chief executive of a mobile payments service called Square, which is located near Twitter's San Francisco headquarters.
"As executive chairman, Jack will dive into work with more than 450 people, led by an experienced executive team," Twitter said in a company statement.
"The timing is fortuitous; not only is Twitter experiencing record growth, but we also now have a new infrastructure that will keep us ahead of that growth and enable us to launch products that will make Twitter more instant, simple and always present," the statement said.
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