A man entered the shop on Main Street at Cross Hills near Skipton between 8.30pm and 9.30pm on Sunday (January 20) and produced a knife with a red handle or blade.
He ordered a member of staff to empty the till before running off with the cash.
No-one was injured in the incident .
A member of the public, who was wearing a white or light coloured coat, walked past the front of the shop as the offender left shortly after 8.45pm.
Police are appealing for that person to get in touch with them.
The robber was wearing a dark jacket, dark trousers and white trainers.
Police said he was also wearing a woolly type hat rolled down into a balaclava.
Call North Yorkshire Police on 101 with information, quoting reference number 12190012176.