A 15-year-old boy was hit in the face during an assault in Grantham.
The incident happened between Dysart Road and The Drive in Grantham at around 3.15pm on March 11.
The boy did not sustain any lasting injuries, police said. Nothing was stolen and the motive for the assault is currently unknown.
The offender is described as being white, around five foot 10 and of slim build.
He was wearing a mask or balaclava obscuring his features. He was wearing a khaki green tracksuit and Nike footwear.
Anyone with information or who saw the incident should contact Lincolnshire Police on 101 or via email at [email protected] quoting reference number 277 of March 11.