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.

At OpenAI, Emmett Shear, the former chief executive of Twitch, will replace Mira Murati as interim chief, 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.

At Microsoft, Satya Nadella, the tech giant’s chief executive, said that Mr. Altman would be chief executive of the new research lab, “setting a new pace for innovation,” in an apparent contrast at the OpenAI board’s desire for caution in developing A.I. technology. Mr. Nadella noted in a post to X, formerly known as Twitter, that Mr. Altman’s new group will operate as an independent entity within Microsoft.

Mr. Nadella left room for other unnamed colleagues who may join the two co-founders at Microsoft. “We look forward to moving quickly to provide them with the resources needed for their success,” he said.

Mr. Altman responded cryptically, writing on X, “the mission continues.” By Monday morning, more than 550 of OpenAI’s 700 employees had signed a letter saying they might quit to join Mr. Altman’s new project at Microsoft unless the start-up’s board resigned, three people who viewed the letter said.

A spokesman for Microsoft declined to comment further beyond Mr. Nadella’s posts to X. The letter was earlier reported by Wired.

Mr. Altman’s firing startled the tech industry and OpenAI’s investors, which include Microsoft, Sequoia Capital and Thrive Capital. Microsoft, which has invested more than $13 billion in OpenAI, only learned of Mr. Altman’s exit one minute before it was announced, while other investors discovered that he had been forced out via social media. They were given no further information or updates over the weekend.

“The board firmly stands by its decision as the only path to advance and defend the mission of OpenAI,” said the memo on Sunday, 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.

“Put simply, Sam’s behavior and lack of transparency in his interactions with the board undermined the board’s ability to effectively supervise the company in the manner it was mandated to do,” the memo said.

The departure of Mr. Altman, 38, also drew attention to a rift in the A.I. community between people who believe A.I. is the most important new technology since web browsers and others who worry that moving too fast to develop it could be dangerous. Mr. Sutskever, in particular, was worried that Mr. Altman was too focused on building OpenAI’s business while not paying enough attention to the dangers of A.I.

The board’s decision to remove Mr. Altman was a shock to industry allies and rank-and-file employees who supported the charismatic founder. Silicon Valley investors and tech executives expressed their support of Mr. Altman and Mr. Brockman. By Friday evening, Mr. Altman was pitching a new A.I. start-up to investors and planned to start the company with Mr. Brockman.

Since OpenAI released its hit ChatGPT chatbot almost a year ago, artificial intelligence has captured the public’s imagination, with hopes that it could be used for important work like drug research or to help teach children. But some A.I. scientists and political leaders worry about its risks, such as jobs getting automated out of existence or autonomous warfare that grows beyond human control.

OpenAI has been the gravitational center of that discussion along with its former chief executive, who has done more than anyone over the last year to make artificial intelligence a mainstream topic.

The board did not cite specific incidents involving Mr. Altman as the cause for removing him. Rather, it claimed that Mr. Altman had “lost the trust of the board of directors,” and that removing him was “necessary to preserve the board’s ability to execute its responsibilities and advance the mission of this organization.”

“It is paramount that any C.E.O. be honest and transparent with his or her board,” the memo said.

OpenAI and Mr. Altman did not immediately respond to requests for comment.

The A.I. company has an unusual governance structure. It is controlled by the board of a nonprofit that can decide the company’s leadership and its investors have no formal way of influencing decisions.

Some OpenAI employees pledged to quit OpenAI or join Mr. Altman’s new potential venture if the board did not relent. But even as Mr. Altman made his pitch for a new company, investors were pushing for the return of Mr. Altman and Mr. Brockman.

Throughout the weekend, Mr. Altman and his supporters pressured OpenAI’s board with appeals from venture capitalists, other tech executives and employees. Microsoft led the charge, three people said, and smaller investors channeled their concerns through Microsoft.

The effort, the people said, was meant to show the company’s board how popular Mr. Altman was among OpenAI’s employees and across Silicon Valley.

The lack of details about the reasons behind Mr. Altman’s ouster emboldened his supporters. Some argued that OpenAI’s nonprofit board could no longer support the business that OpenAI had become — one with 700 employees, numerous customers and corporate partnerships that is on track to post $1 billion in annual revenue.

Mr. Altman, Mr. Brockman and Mr. Sutskever created OpenAI in 2015 alongside nine others, including Elon Musk, the chief executive of Tesla. The group founded the A.I. lab as a nonprofit, saying that unlike Google and other tech giants, it would not be driven by commercial incentives.

In 2018, after Mr. Musk parted ways with OpenAI, Mr. Altman transformed the lab into a for-profit company that is controlled by the nonprofit and its board. Over the next several years, he raised the billions of dollars the company would need to build technologies like ChatGPT.

Before joining OpenAI, Mr. Shear led Twitch through its transformation from an upstart platform called Justin.tv to a behemoth that was acquired by Amazon in 2014. He stayed on after the tech giant took over, and only departed earlier this year, saying he was having a child.

Mr. Shear, a longtime video gamer, was viewed as a competent leader at Twitch but had his critics. He was perceived to be too focused on cost-cutting and turning the money-losing site into a more profitable business.

“We apologize for the abruptness of the process that we felt was required by the situation,” the board said in its memo. “Even understanding the questions it has raised, we continue to believe our actions were necessary.”

On Monday, as OpenAI employees signed the letter saying they might leave to join Mr. Altman’s new project at Microsoft, one name stood out: Mr. Sutskever. He posted a message to X saying he deeply regretted his role in the board’s decision.

“I never intended to harm OpenAI,” he said. “I love everything we’ve built together and I will do everything I can to reunite the company.”

Kellen Browning, Karen Weise, Erin Griffith and Tripp Mickle contributed reporting.