A 16-year-old boy received a cut to the hand while he was in Lings Way, Northampton on Friday night.
The teenager was approached by a man who assaulted him, causing the injury to his hand, sometime between 9.30pm and 10.15pm, on Lings Way, between the bus stop by Crestwood Road and the traffic light crossing.
The offender was a man, aged 18 to 25 and he wore all black clothing, including a balaclava.
The victim required stitches for the cut.
Police would like to speak to anyone who may have been in the Lings Way area of Northampton on Friday night and witnessed this incident or saw anyone matching the description of the offender.
Witnesses or anyone with information is asked to call police on 101 or Crimestoppers anonymously on 0800 555 111.