Just before 6am the man entered the service station on Aberdare Road, threatening a 31-year-old staff member, before taking cash and cigarettes and fleeing onto a dirt bike where a second man was waiting.
The bike was last seen riding along Aberdare Road.
Police are now appealing for anyone who may have seen or heard anything to come forward.
Both men were last seen wearing motorbike helmets, with dark clothing and their faces covered.
