Officers were called shortly before 16:00 GMT on Sunday following reports of an attack in Bugs Bottom fields, Emmer Green.
The boy was pronounced dead at the scene and police are appealing for witnesses to come forward.
No arrests have been made, but Thames Valley Police believe the boy was attacked by two or three males.
Police said the boy's next of kin have been informed, but no formal identification has taken place.
A "considerable police presence" will be in place for several days, police said.