In a surprise blog post overnight (Australian time) the two co-founders announced that they would step away from day-to-day management of Alphabet, which includes Google and many other projects.
Google chief executive Sundar Pichai will retain his current role and will also become chief executive of Alphabet.
In their blog post, Page and Brin wrote, "With Alphabet now well-established, and Google and the Other Bets operating effectively as independent companies, it's the natural time to simplify our management structure. "We've never been ones to hold on to management roles when we think there's a better way to run the company. And Alphabet and Google no longer need two CEOs and a president."
"The founders have given all of us an incredible chance to have an impact on the world. Thanks to them, we have a timeless mission, enduring values, and a culture of collaboration and exploration that makes it exciting to come to work every day. It's a strong foundation on which we will continue to build. Can't wait to see where we go next and look forward to continuing the journey with all of you."
In a letter to investors, John Hennessy, chairman of Alphabet's board, said: "It's impossible to overstate Larry and Sergey's contributions over the past 21 years. I'm grateful that they will continue their involvement on the board."
Shares in Alphabet closed the day marginally higher after an initial drop as the news was announced.