The incident took place at the Iceland store in Corporation Street at about 5.30pm on Saturday, October 20. Police have released details and the image today (November 27).
A man stole meat from the store and when he was challenged by staff he pulled out a knife.
Officers are keen to locate the man pictured who they say could assist the investigation.
He, or anyone who knows him, is asked to contact Northamptonshire Police on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555111.