Ventura sheriff's officials said the attack happened about 8:30 a.m. in the 3400 block of Citrus Street in El Rio.
The boy was on a bicycle when the 100-pound dog came out of a neighboring yard and attacked him, officials said. He then ran to his cousin's house for safety.
The boy was taken to Ventura County Medical Center.
The dog's owner told deputies that the dog was secured in the backyard. At that point, deputies said the dog came out again, but was not aggressive toward them.
The owner grabbed the dog and placed it in a patrol car. Animal control then put the dog into quarantine.
Authorities are investigating to see if the owner is at fault.