The incident, which was captured on surveillance video, happened at a shop on Foothill Boulevard and Euclid Avenue the day after Christmas, according to the Upland Police Department.
After pulling out the knife, police say the man demanded money from the cash register and ordered the worker to her knees before he took off.
Police described the suspect as being around 40 years old, 5 feet 10 inches tall with a thick mustache and goatee.
Anybody with information is urged to contact detectives.