The incident took place between 4.20pm and 4.25pm on Thursday, February 21.
A 13-year-old boy was playing in a park near Brinkhill Walk when he and his friends were approached by a group of boys.
One of them racially abused the boy before he then grabbed his clothing and headbutted him in the face, causing bruising and swelling.
The suspect is described as a white boy aged around 12, about 5ft tall and with blond hair.
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A police spokesman said: “Anyone who saw the incident or has information about it is asked to call Northamptonshire Police on 101.
“Information can also be shared with Crimestoppers in confidence on 0800 555111.”