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.
Detective Superintendent Kevin Brown from Thames Valley Police said: "This is a tragic and shocking incident which has resulted in the death of a young boy.
"We have specially trained family liaison officers supporting the boy's family and the thoughts of all of us within the force are with them at this extremely distressing time."
Police are appealing for anyone who was in the area between 15:00 GMT and 16:30 GMT who might have taken photos or camera footage to contact them if they notice anything suspicious.
Det Supt Brown said: "This is an area that is used extensively by dog walkers, and I believe that there will have been witnesses to this dreadful incident, or who may have seen the offenders leaving the area quickly."