SAN FRANCISCO—Microsoft said it is hiring Sam Altman to helm a new advanced artificial-intelligence research team, after his bid to return to OpenAI fell apart Sunday with the board that fired him declining to agree to the proposed terms of his reinstatement.

Microsoft Chief Executive Satya Nadella posted on X, formerly known as Twitter, late Sunday that Altman and Greg Brockman, OpenAI’s president and co-founder who resigned Friday in protest over Altman’s ouster, will lead its team alongside unspecified colleagues. Nadella said Microsoft was committed to its partnership with OpenAI and that it would move quickly to provide Altman and Brockman with “the resources needed for their success.”