Sheriff Dave Lucas says about nine cars were entered in the Mount Olivet area outside Barnesville.
He says credit and debit cards and cash were stolen from the vehicles.
He says only one vehicle was damaged by forced entry; its window was broken out.
The others were apparently unlocked.
"You know, a lot of people leave their cars unlocked to prevent someone from breaking a window," said Sheriff Lucas. "And that's your choice. And I can understand that. But if there's nothing in there they can steal, they won't get anything. So my advice to the people of Belmont County is do not leave any valuables in your car."
He says it takes only a moment to grab your valuables and take them into the house with you.
The sheriff says his officers are investigating this rash of car break-ins, and they believe it's the work of one or two people.
Anyone with information is urged to call the sheriff's department at (740) 695-7933.