The dangerous encounter happened approximately 200 yards from shore. The girl was with a group of paddleboarders in a remote part of the island, about a mile from Avalon.
The girl was not hurt, but her board was bitten several times.
"We like to say that the shark's investigating, but in reality, it's a shark attack," said marine biologist Luke Tipple.
Tipple, who documents sharks for a living, says the shark responsible for the attack could have been a Mako, but was probably a young Great White. Right now, a lot of Great Whites are in area waters because they are making their way south to the Sea of Cortez.
"A very large Mako could cause the same type of damage. They don't typically fall under the same attack pattern," Tipple said.
Swimmers and divers are being advised to use caution in waters around Catalina Island, but experts say shark attacks in that area are rare. Officials said they will continue to maintain constant patrols throughout the area.