It happened at the Rite Aid on Key Parkway and Baughman Lane.
Police say the man threatened the clerk with a knife, and stole a television.
The man is described to be about six feet tall, wearing a trench coat, with a yellow shirt underneath.
The suspect was last seen leaving the area on foot.
"Well we were informed that the suspect took a television, placed it under his trench coat, and while leaving he pulled out knife and threatened the clerk and then subsequently left the store on foot," says Thomas Tokarz, with the Criminal Investigations Unit.
Anyone with information is asked to call the Frederick Police Department at (301)- 600-2101.