The board of directors of OpenAI, the high-flying artificial intelligence start-up, said in a note to employees on Sunday night that its former chief, Sam Altman, would not be returning to his job, while naming his second interim replacement in two days.

Hours later, in another head-spinning move, Microsoft said it was hiring Mr. Altman and Greg Brockman, OpenAI’s president and a company co-founder who quit in solidarity with Mr. Altman. The two men will lead an advanced research lab at Microsoft.

Emmett Shear, the former chief executive of Twitch, will replace Mira Murati as interim chief executive of OpenAI, the board said. Ms. Murati, a longtime OpenAI executive, had been appointed to that role after Mr. Altman’s ouster on Friday. The board said Mr. Shear has a “unique mix of skills, expertise and relationships that will drive OpenAI forward,” according to the memo viewed by The New York Times.

“The board firmly stands by its decision as the only path to advance and defend the mission of OpenAI,” said the memo, referring to Mr. Altman’s removal from the company on Friday. It was signed by each of the four directors on the company’s board; Adam D’Angelo, Helen Toner, Ilya Sutskever, and Tasha McCauley.