The robbery happened at 5.20am on Wednesday morning at the post office in Flaxley Road.
A man, wearing a balaclava burst into the post office and threatened the man working inside.
He demanded cash from the till, before fleeing with a quantity of stolen money.
Police said the suspect was wearing a cream-knitted beanie hat, and a black jacket with two stripes down each arm.
Officers are appealing to anyone who was in the area at the time who may have witnessed anything to contact police on 101, quoting crime reference number 12170038913.