A girl was walking across Delapre Recreation Ground when she was attacked and cut on the hand with a knife.
The teenager, 16, was walking across the recreation ground between 6.50pm and 7.15pm on Friday evening when she was approached by a group of four young men who bumped into her, cutting her hand with what is believed to have been a knife.
The offender ran off towards Towcester Road and the teenager was taken to hopital with a minor hand injury.
He was wearing a black hooded top with the hood up, and a scarf covering most of his face. He was white, about 16 or 17 years-old, of thin build and about 5ft 8ins. He wore black tracksuit bottoms.
Police would like to speak to anyone who may have witnessed this incident or who may have any information regarding it.
Witnesses, or anyone with any information, can call Northamptonshire Police on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111 or use the anonymous online form at www.crimestoppers-uk.org.