Medford Police said the incident occurred at the Short Stop Market on the 3200 block of South Pacific Highway around 4:30 in the afternoon.
A suspect who was thought to have a gun was attempting to rob the clerk when a customer in line jumped into action.
Police said the citizen grabbed the suspect in a choke hold and threw him to the ground. The citizen and others held the suspect down until officers arrived.
MPD is still investigating the case and will release more information after the suspect is booked in jail.