A 14-year-old boy was assaulted while walking along Finedon Road in Wellingborough.
The incident happened between 9.50pm and 10.05pm on Sunday, October 22, when a man on a bicycle punched the victim before cycling off towards the Hemmingwell estate.
The offender is described as white, aged about 17 and of slim build.
He wore a black and white Nike tracksuit and a hi-vis vest and was riding a red and orange bicycle.
Witnesses or anyone with information about the assault can contact Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555111.