The man demanded bar staff hand over cash at the Maiden Place Social Club, off Kilnsea Drive, while he brandished a rounders bat on Sunday.
The robbery took place at 8.40pm as the man ordered a female member of staff to empty a safe of the bar's takings.
Cash was handed over to the man who then fled the scene.
The man was described as white, 6ft 2ins tall, with blonde hair and spoke with a local accent.
A Thames Valley Police spokesman said: “He [the offender] was carrying a bat, which appeared to be similar to a rounders bat, and demanded a female member of staff to open the safe."