The incident happened Thursday night at a Chevron station in the 2100 block of East McFadden Avenue.
Police say the suspect aimed a handgun at the clerk and ordered her to give him all the money from the cash register.
The suspect was described as a white male in his 40s with black hair and a beard. He was last seen wearing a black sweater.
Investigators say he made off with nearly $700.
If you have any information about this incident, you're urged to contact Orange County Crime Stoppers at (855) TIPS-OCCS.