The incident happened at the McColls store at The Island, at about 9pm on Friday, September 11.
The masked man entered the store with the knife and demanded money from two female members of staff.
The man fled the scene on foot in the direction of Plumptre Way, leaving two members of staff and one customer shaken but otherwise unharmed.
Anyone with information is urged to call Nottinghamshire Police on 101, quoting incident 845 of September 11.
Alternatively call Crimestoppers anonymously on 0800 555 111.