CCTV images have been released by South Yorkshire Police after an unknown offender entered the shop on Market Street at around 5.30pm on Friday October 25, demandng the shop assistant opened the till.
The man claimed he had a gun.
Detective constable Steve Ramsbottom, investigating, said: “Bravely she refused and managed to lock herself in the shop toilets to call 999, staying there until officers arrived. It is reported that the offender left the store empty handed and went in the direction of Pitt Street.
“This has understandably been a highly distressing incident for this young woman. Our enquiries are continuing, but we have now released a CCTV picture of a man that we think could hold information about what happened.
“If you recognise him, please call 101 or contact my team on 01226 736476 quoting crime reference number 14/162049/19.”
Anyone with information can also call Crimestoppers anonymously on 0800 555 111.