It happened at Tony's Mart in the 2000 block of Ramsey Street around 3:45 p.m. Monday.
Police say the suspect walked into the business and opened the cash register while the clerk was not looking, and stole some cash.
The attendant caught the man in the act and attempted to stop him, but he pulled out a large knife and reportedly threatened the woman.
The suspect is described as a black male, approximately 5 feet 5 inches tall and weighing about 140 pounds. He was last seen wearing black cargo style pants, a dark T-shirt and a black baseball cap.
Anyone with information concerning the robbery or the identity of the suspect is asked to contact the Fayetteville Police Department at (910) 433-1856 or Crimestoppers at (910) 483-TIPS (8477).