The suspect was riding a bright blue mountain bike when he started talking to two other teenage boys on the Ripley to Bilton cycle path near the A61 Ripley.
He asked to look at their mobile phones, then cycled off towards Bilton with one of the phones.
The teenager police want to trace is described as being 16-18 years old, between 5ft 6in and 5ft 9in, and slim. He had a shaved head and was wearing a navy coloured jumper, grey jogging bottoms and white trainers. He was carrying a navy coloured Armani man bag.
The incident took place on the Ripley to Bilton cycle path near to the Killinghall Bridge A61 at 7.15pm on Saturday, August 27.
A police spokesman said; “We are conducting enquiries and are requesting the public’s assistance to help identify the suspect involved in the incident.
“In particular, we are appealing for information about the identity of the suspect and his movements at the time in question and any information that may assist in confirming this.”
Call 101, select option 2, and ask for Pc 1519 Darren Copley, quoting 12160155667.
If you wish to remain anonymous, you can pass information to Crimestoppers on 0800 555 111.