Officers say it happened at the McDonald's on Dual Highway just after 11 p.m. Wednesday night.
Witnesses told police a man armed with a semi-automatic handgun entered the store and demanded money from a cashier.
Police say the man was able to leave the scene on foot with an undisclosed amount of money. A canine search was conducted, but the suspect has yet to be found.
The robber is described as a black man, wearing a red fabric face mask, with a green t-shirt and jeans.
Anyone with information is asked to call the Hagerstown Police Department at (301)-790-3700.
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