Police said the incident occurred around 2:50 p.m. at International Food Market in the 3500 block of Laurel Fort Meade Road.
The man then demanded cash from the clerk, who complied by handing over an undisclosed amount of cash, police said.
Police said the man then exited the store and fled in a gray sedan, which was also occupied by another person.
Police said they believe the sedan may be a Ford Escort.
On Aug. 25, the market was robbed in similar fashion by a man with a handgun, police said.
Anyone with information can call Metro Crime Stoppers at 1-866-7LOCKUP or text "MCS" and a message to 274637.