The incident took place on 4 February at a supermarket on Hyndburn Road, Accrington.
Lancashire Constabulary described the incident as "completely unacceptable".
In a statement, the police said the 52-year-old victim was working behind the fish counter when a young woman approached to ask about different fish.
The boy is described as being about the same age The boy is described as being about the same age as the girl
"Without warning, the woman has picked up a large bream from the fish stall and slapped the worker across the face before running out of the store," said the statement from Lancashire Constabulary.
"The victim believes the incident was being filmed on a mobile phone by a man stood nearby who also ran from the scene when challenged."
The girl is described as being Irish and aged between 15 and 18 with "blonde, shoulder-length untidy hair".
She was wearing dark jeans and a blue coat with white cuffs, white down the front and a red collar with 'I love PB' on the left breast pocket.
The boy is described as being about the same age, 5ft 8 tall, wearing a dark coat and dark grey trousers and boots.
PC Graham Hartley said: "This behaviour is completely unacceptable and I would appeal to anybody that witnessed this incident or recognises either the man or the woman pictured in the CCTV images to come forward."