The teenager was walking along King Street in Paisley, Renfrewshire, when a man stopped him and asked to borrow his mobile phone.
He then punched the boy in the face and ran off.
The victim, who did not need hospital treatment, then told two police officers out on patrol what had happened to him.
Police are appealing for information about the robbery which took place near to the railway bridge at the junction with Douglas Street at around 6.30pm on Wednesday.
The suspect is described as white, 5ft 5in, with dark short hair and was wearing a black hooded top with an Adidas logo on the front.
Detective Constable Craig Stirling from Paisley CID said: “This was a callous and opportunistic attack on a teenage boy and it is vital that we trace the individual responsible.
“I would appeal to anyone who was in the area yesterday evening who either witnessed what happened or noticed a man matching the above description acting suspiciously, to please get in touch.”
Anyone with information is asked to contact Paisley CID via 101, quoting incident number 3177 of Wednesday February 7 2018.
Alternatively people can call CrimeStoppers on 0800 55 111 where details can be given in confidence.