A 44-year-old man was assaulted as he left Costcutter in Kettering Road, Northampton, on Saturday night.
The victim was punched in the head, causing him to fall to the floor during the incident that took place at about 11.15pm.
The offender was a man with short black hair, olive coloured skin and wore denim knee length shorts and a burgandy body warmer.
Police would like to speak to anyone who may have witnessed this incident or who may have any information regarding it.
The victim required hospital treatment as a result of the assault.
Witnesses or anyone with information is asked to contact police on 101 or Crimestoppers anonymously on 0800 555 111.