A man is alleged to have brandished a knife at a member of staff in the B&M store in Whitworth Way, Wellingborough, before demanding money.
A police spokesman said no money was handed over and the man left.
The incident was reported to have happened shortly after midday on Sunday, August 21.
An 18-year-old man from London was arrested in connection with the incident and has since been released on bail.
Officers are appealing to anyone who was in or around the store at the time to contact police.
Anyone with information can contact Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555111.