Police are appealing for witnesses following the robbery which happened on the path off Grainger Drive, between Acacia Drive and Westmoreland Mews, in Leegomery.
The boy was approached by three people who threatened him, one was in possession of a knife.
The first suspect was male and wearing a black balaclava, black jumper, white T-shirt and black trousers.
The second was a teenage girl described as white, aged around 16 with long black hair and was wearing a white T-shirt and black trousers.
The third suspect is described as white, around 16-years-old, and was wearing a black and white jumper and black tracksuit bottoms.
Police say they took a mobile phone and small amount of money.
The robbery happened at about 4.30pm yesterday.
Anyone with information, or any motorists who were in the area and have dash cam footage, are asked to call police on 101 quoting reference 0511S 160919.
Alternatively, contact Crimestoppers anonymously on 0800 555111 or at crimestoppers-uk.org