The man, who is described as white, aged about 25 and looking very pale and gaunt, snatched the money from the till and then ran from the Cliftonville Inn pub on George Street.
Police said he walked through the toilets at the back of the pub and waited until a member of staff opened the till. He was wearing a dark blue or black raincoat with the hood up.
Licensees were warned to be on their guard against crime after a pub was scammed out of £100 by fraudsters earlier this year.
Conmen confused bar staff at the Royal Oak in Chorlton, Manchester, by asking for £200 to be changed into 20p coins.