PC Paul Kirwin wrestled the gun off the man who was pointing it at terrified shop staff in Litherland in July 2013.
He had spotted the suspect acting suspiciously in an alleyway running between shops and a derelict building while on patrol with a colleague.
Peter Singleton, chairman, of the Merseyside Police Federation, praised PC Kirwin's "quick thinking".
PC Kirwin saw the man leaning through a door at the rear of one of the shops, pointing a handgun at terrified staff members.
The officer quietly walked up behind him and grabbed his arm before a struggle ensued.
PC Kirwin wrestled with the man ensuring that he could not point the firearm at either himself or shop staff.
He managed to keep the man restrained and was able to force him to drop the handgun inside the shop.
At this point, PC Kirwin was joined by his colleague and formally cautioned and arrested the man for attempted armed robbery, and possession of a firearm with intent, offences that he later admitted in court.
He was presented with the award by shadow home secretary Andy Burnham at a ceremony in London on Tuesday.
A Merseyside Police officer who disarmed and arrested a robber with a handgun has won a Police Bravery Award.