The authorities have released two images of a person they think could have vital information on the robbery.
A teenager was walking on Highbridge Walk on June 22 when he was threatened by someone carrying a knife.
At roughly 5:45pm in central Aylesbury the offender yelled at the victim demanding he hand over money and his phone.
The victim complied giving over his phone.
He was left uninjured after the startling incident.
Investigating officer, Detective Constable Sarah Spain, based at Aylesbury police station, said: “We are releasing these images as we believe the individual in them may have information about what happened.
“If you recognise who this is or if it is you pictured, please get in touch.
“Anyone with information can contact police by calling 101 or making a report online, quoting reference 43220274093.
“Alternatively, if you wish to remain anonymous, you can contact the independent charity Crimestoppers by calling 0800 555 111 or online.”