The director of Kew Gardens will step down next summer, the world famous botanical organisation has said.
Sir Peter has been director of Kew for seven years
An emotional Peter Crane announced his decision to staff on Friday saying he was taking up an academic post at the University of Chicago.
He has been in charge for the past seven years. During that time Kew Gardens, in west London, has been given World Heritage Site status.
A Kew spokesman said the search for his replacement would begin immediately.
Sir Peter is also responsible for Kew's country estate at Wakehurst Place in West Sussex, which is home to the £80m Millennium Seed Bank Project.
It was here that he announced his decision to leave.
"Kew has been at the centre of my life for the past seven years", he said.
"It is very hard to let go. However, world-leading organisations like Kew need a regular infusion of new ideas and energy to stay at the top of their game.
Increased global profile
"This is also the right time for me to return to Chicago."
Before his appointment at Kew, Sir Peter spent 17 years in Chicago as the director of the Field Museum of Natural History.
Lord Bach, minister with responsibility for the Royal Botanic Gardens, Kew, said: "The inscription of Kew as a World Heritage Site, is the most tangible of his accomplishments.
"This however has to be seen in the context of the increased global profile both scientifically and educationally of this world renowned institution.
"Sir Peter has encouraged greater access - for education, science and pleasure - to the Royal Botanic Gardens sites at Kew and Wakehurst Place.
"The government places great emphasis on this aspect of his achievements which is of benefit to over 1.5 million visitors annually."