Officers were called to the Fastrip convenience store on Clinton and Chestnut just after 5:30 a.m. Wednesday.
The worker told them a man wearing dark clothing and a facemask went into the store and demanded money from the register.
The worker handed over the cash and the man ran off with the money and stolen liquor bottles.
Before leaving the store, he fired one shot into the ceiling, causing damage to the business.
There was no one else inside the store at the time of the robbery - and the worker was not injured.
Detectives are reviewing surveillance video from the store in an attempt to identify the gunman.