The incident occurred at around 4.15am on September 2 on Market Street after the 27-year-old victim and the offender became involved in a verbal altercation.
The victim was punched to the face by the offender, causing him to fall and hit his head on the floor.
He was knocked unconscious during the incident and was later treated at Royal Preston Hospital for a head injury.
Police would now like to speak to the man in the attached CCTV images as part of on-going enquiries.
PC Leanne Oxley, of Chorley Police, said: “The victim has been left quite shaken by this incident and our enquiries are on-going to establish exactly what happened.
“We would now like to speak to the man in the CCTV images as we believe he may be able to help with our investigation. If you know who he is, or indeed, if you think this may be you, we would ask you to contact us as soon as possible.
“Finally, I would urge anyone who may have witnessed this incident to come forward as you may have vital information.”
The man police would like to speak to is described as white, around 6ft 2ins tall, and of large build. In the CCTV pictures he is seen wearing a white t-shirt and jeans.
Anybody with information is asked to contact police on 101 quoting log number 288 of September 2.