At around 5.40pm, a 16-year-old girl was approached in the underpass which links Hazeley School and Oakhill, passing under H5 Portway.
The victim was threatened and then punched in the face. She fell to the ground and the offender stole her BlackBerry mobile phone and ran away. The victim suffered bruising to her face and a cut lip. No hospital treatment was required.
The offender is a white male, around 17-years-old, 5ft 8ins, of a slim build, with short brown hair and a southern English accent. He was wearing light blue jeans, a black hooded top and a pair of black and white Nike trainers.
Det Con Wendy Cissell from Milton Keynes police station said: “The offender is known to the victim.
“I am appealing for witnesses who may have seen anything suspicious or who saw the offenders in the area at the time to come forward.”
Anyone with information should contact Det Con Cissell via the Thames Valley Police Enquiry Centre on 101.