A female jogger had her bum pinched as she ran past a group of teenagers in the Racecourse park in Northampton.
The woman challenged the group after the incident but they ran off in the direction of Colwyn Road.
They are described as Asian, aged between 12 and 14 years old and were dressed smartly.
It happened on Sunday between 4.45pm and 5pm.
Any witnesses, or anyone with information, can call Northamptonshire Police on 101 or information can be given anonymously to Crimestoppers on 0800 555 111.