A woman, who was walking in Daventry, was approached by a girl who shouted at her from behind, before punching her to the side of the face, causing swelling and pain.
The incident happened at around 1.30pm on Sunday, April 30 in The Inlands, Northamptonshire Police today revealed.
The offender then left the scene with another female.
A spokeswoman for Northamptonshire Police said: "The suspect is described as white, around 16 years of age, ‘tom-boyish’ cropped hair and wearing a baseball cap."
Anyone with information can contact Northamptonshire Police on 101. Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.