Police say a man wearing a balaclava and armed with a large knife entered McLeans Bookmakers on Blucher at around 7.30 pm.
After demanding money from the tills, he made off on foot with an undisclosed amount of cash.
The robber is described as being about 6ft tall and was wearing a dark coat and tracksuit bottoms.
Investigating officer Detective Sargeant Brian Reid appealed anyone with information about the man, or any other suspicious activity in the area, to call police at Strand Road on 101 non emergency number.
Or alternatively information can be passed to Crimestoppers anonymously on 0800 555 111.Tags: