The boy relinquished his bike after being approached by a group of men at around 3:35pm.
One of the men was carrying a kitchen knife in his back pocket.
He was on New Street when he demanded the bike, they were close to the junction of Cambridge Place.
After grabbing the bike the gang headed off to Buckingham Street via the nearby alleyway.
Witness reports describe one of the offenders as aged between 20 and 40, white, approximately five foot seven inches tall, and of average build.
He was wearing a black short-sleeved top and grey denim shorts.
The victim’s bike is a turquoise Challenge Crusade 26-inch Rigid men’s bike.
Investigating officer, Detective Constable Mandy Cutler, based at Aylesbury police station said: “I am appealing to anyone who may have witnessed the offence, or has footage from the area to please contact the force.
“The easiest way to provide us with information is on our website or by calling 101, quoting investigation reference 43220265646.
“Alternatively, you can provide information anonymously to the independent charity, Crimestoppers, by calling 0800 555 111 or via its website.”