She was on the Animal Kingdom's Dinosaur ride with her grandson when she found the weapon. She then handed it over to a park attendant.
The gun's owner, Angelo Lista, has a concealed weapons permit and told authorities he discovered his pistol missing several minutes after leaving the ride.
Disney World doesn't allow weapons of any kind, whether someone has a permit or not. The gun owner said he didn't know people weren't allowed to bring in guns.
Lista said he thought the security checkpoint at the entrance was just to check for bombs and explosives.
Disney is the parent company of ABC7.
The Associated Press contributed to this report.