Andy Rubin, who co-founded the Android project, is to step down after around nine years at the technology giant.
Announcing Mr Rubin's departure, Google said he created "something truly remarkable" in Android.
Mr Rubin, who has also worked for Apple, reportedly plans to found an incubator for hardware start-up firms.
"I want to wish Andy all the best with what's next. With Android he created something truly remarkable - with a billion plus happy users. Thank you," said Google's chief executive and co-founder Larry Page.
Mr Rubin's departure comes after he stepped down from Google's Android division in March 2013 to begin working on the technology giant's robotics project.
He was behind the acquisition of seven robotics companies in a six-month spell as part of a reported attempt to develop a rival to Amazon's mooted drone delivery system. Google said that those claims were speculation.
When he left, Android came under the control of Sunder Pichai, a rising star at the company who worked with Google Chrome and Apps, and who has since picked up yet more responsibilities from Larry Page.'Extraordinary progress'
Writing on Google's blog at the time, Mr Page wrote that when Andy Rubin he first launched Android in 2003, "most people thought he was nuts".
He said Mr Rubin's vision of "aligning standards around an open-source operating system" struck a chord with Google because it simplified the development process.
And he lauded its subsequent success, which he said was "pretty extraordinary progress for a decade's work".
The move comes amid a series of executive changes at Google, which the New York Times reported were designed to relieve Larry Page of the responsibility of running individual projects.
"These changes will free me up a bit so I can focus on the bigger picture with Sundar [Pichai] when it comes to our core products," it quoted a memo from Mr Page as saying.
Mr Graf was replaced by Kevin Weil, who took up the role of vice president of product. The former retained his title but was given a narrower role.
He will be expected to work on geolocation features and other strategic initiatives, a source intimated.
His demotion came only six months after he was hired from Google and is the second time Twitter's head of product has been replaced in the year since the initial public offering.