It happened at the Tobacco Outlet in the 3200 block of North Main around 8:30.
Details are still limited, but police say they believe the suspect, armed with a handgun, entered the store and demanded money.
They say he took off on foot with an undisclosed amount of cash.
Police describe the suspect as a black male around 30 years old, about 5'4 in height.
No one was hurt.
Anyone with information is asked to call Rockford Police.