The assault was reported to the police two days ago but the attack happened around March 23.
Thames Valley Police said the girl had been on her own when a man pulled her to the woodland area raped her and then fled.
Police said the offender is described as a white man aged around 45 and a skinny build.
Investigating officer Justin Fletcher said: "We would urge anyone who saw the victim or a man answering this description in the area around that time to contact us as a matter of urgency. While this was a very traumatic incident for the victim, it is also very rare."