Dick Costolo will step down as CEO of Twitter after almost five years. It did not disclose a reason for his departure.

Twitter Inc. announced the move in a press release Thursday and Costolo also tweeted the news through his own account.

The San Francisco company says co-founder and Chairman Jack Dorsey, 38, will be its interim CEO while it looks for a replacement.

Costolo, 51, will stay on Twitter's board of directors. He has been a director since September 2009 and became CEO in October 2010.