A masked man brandished a handgun as he demanded a Post Office worker hand over cash.
Police are appealing for information after the man entered Bugthorpe Post Office yesterday (November 3) at around 12.35pm.
Humberside Police say he was carrying what was suspected to be handgun and was wearing a mask.
The worker raised the alarm after locking himself inside a private room in the shop and the offender left the scene.
It is not known whether anything was stolen.
Detective Inspector Andy Robinson, who is leading the investigation, said: “We are working in partnership with our colleagues in North Yorkshire Police to trace the man responsible.
“I would appeal for anyone who was in Bugthorpe between 11am and 2pm to get in touch. They may have vital information about the incident and we can also eliminate them from our enquiries.”
The offender is described as wearing black clothing – including a waterproof jacket – and a mask over his face. He was around 5ft 8in tall and of average build.
Anyone with information is asked to call 101, quoting log number 228 of November 3.