Google CEO Sundar Pichai, who previously had been a member of the Alphabet board of directors, will head both companies moving forward.
"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," Page and Brin wrote in a blog post.
"We've never been ones to hold on to management roles when we think there's a better way to run the company," they added.
Alphabet, created in 2015, comprises several companies beyond Google, including the driverless car firm Waymo and the biotech company Calico.
Page and Brin have shied away from the spotlight while heading Alphabet, rarely appearing at events or speaking on earnings calls.
According to Tuesday's blog post, they will "remain actively involved as Board members, shareholders and co-founders."