Telford Police are appealing for witnesses to the incident at Asda in Donnington, at around 10pm on Friday.
Police said that the boys were threatened for their phones and belongings.
A spokesman for the police said: "At around 10pm on Friday night two teenage boys were approached in an alleyway near the supermarket.
"A 16-year-old boy and a man believed to be 18 threatened the two boys with a knife and demanded their belongings and phones before leaving the scene.
"The older of the two had dark hair, around 5ft 8ins tall, of a slim build, he wore a dark jacket, the second also of a slim build around 5ft 10ins, he was wearing a grey tracksuit."
Anyone with information can contact DS Bentley from Malinsgate Police Station on 101 quoting reference 862_i_24072020.