The company said Altman was "not consistently candid in his communications" with the board.
Greg Brockman, OpenAI's president, will step down from the board but remain at the company.
OpenAI said its board consists of OpenAI's chief scientist, Ilya Sutskever, and three non-employees.
OpenAI's key business partner, Microsoft, said that the transition won't affect its relationship.
Altman has a number of possible next steps.
He placed large bets on several other ambitious projects. share.