The robbery happened in Weedon Road, Sixfields, on December 22.
A spokesman for Northamptonshire Police said: "Between 8.45pm and 9pm, the 18-year-old victim was walking along the path behind Frankie and Benny’s when he was attacked by two, possibly three, men. One of the men hit the victim and stole his bag and mobile phone.
"The men were white, aged 18 to 20 and were all wearing hooded tops," he added.
Witnesses, or anyone with information, are asked to call Northamptonshire Police on 101. Alternatively, they can call independent charity Crimestoppers anonymously on 0800 555111.