It is understood the robber threatened the worker with a gun before making off with an unknown amount of money in South Lanarkshire this morning.
Detectives launched a hunt for the suspect after the daylight robbery outside a Scotmid shop in Uddingston.
Officers were called to the scene in Old Edinburgh Road after 9am.
The busy street remained cordoned off while police examined a security van and the surrounding area.
It is understood no-one was injured, and the exact amount stolen in the raid is not yet known.
A police spokeswoman said: "Inquiries are ongoing."
Anyone with information is asked to call 101.