The man entered the store at around 9.30pm and demanded a member of staff hand over money, which he stuffed into a plastic bag before leaving.
No one was injured during the raid and the robber is described as black, wearing a grey hooded tracksuit top and bottoms and black trainers.
Investigating officer, Det Con Nicola Slater of Force CID based at Reading police station said: “If anyone recognises the description of the offender or has any other details about the offence, perhaps you saw something suspicious in the area on Wednesday then please contact me via 101."
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