The incident happened in the car park of the Co-Operative supermarket in Hull Road, Hedon on Friday 21st September 2012 at around 9:00pm.
Two youths aged between 14 and 17 years were seen to steal items from the store and were pursued into the car park by the security guard.
The guard detained one of the youths at the edge of the car park who then assaulted the guard by spitting at him and punching him to the face.
Members of the public approached as the incident happened and it is those people who may be able to help police with their enquiries.
If you witnessed the incident and are able to provide any information to police please call the non emergency number 101 quoting crime reference number 1931751 or Crimestoppers anonymously on 0800 555 111.