Youngsters were threatened with a knife and robbed as they walked through a park on Saturday night.
A group of friends aged between 15 and 20 years old were approached by a man who threatened them with a large kitchen knife and demanded they hand over their mobile phones.
It happened between 8.30pm and 9.30pm in Middlegreen Park near Maple Dean, Stacey Bushes.
The offender, who is described as a black man between 5ft 10ins and 6ft 3ins tall, left with an iPhone 4S and a Sony Eriksson.
He was wearing a hooded top and black padded jacket with a diamond criss cross pattern and coloured jogging bottoms.
Anyone with information should contact Det Con Susan Russell from Milton Keynes police station via the Thames Valley Police Enquiry Centre on 101.
If you don’t want to speak directly to the police, you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or online at Crimestoppers-uk.org
No personal details are taken, information is not traced or recorded and you will not go to court.