Police responded to the Express Lane Gas Station and Convenience Store, 17200 Robbins Road, at about 2:55 a.m.
Employees said a black man with an unknown woman entered the store, pulled out a handgun and ordered the female clerk to to open the cash register.
The man ushered the clerk to a back room and tied her to a chair using duct tape, said Sgt. Matt Wildfong of the Ottawa County Sheriff's Department. The woman stayed in the store area and emptied the cash register.
The suspects fled with an unknown amount of money.
A K-9 unit was used to track the suspects, but lost their scent after a short distance.
Anyone with information on the incident is asked to call Silent Observer at 1-877-88-SILENT, or the Ottawa County Sheriff's Department.