The cat's heroic feat was captured by home security cameras which show the boy riding his bicycle in front of his home in Bakersfield, California.
While the boy, Jeremy, was riding his bicycle, the neighbour's dog, an 8-month-old labrador/chow mix, sneaked up behind him and bit his leg.
The family's cat, named Tara, rushed to Jeremy's rescue and chased the dog away.
Erica Triantafilo, Jeremy's mother, was nearby watering a tree when she heard his screams.
"I didn't really even realise what had happened until after when my husband showed me the surveillance video," Erica was quoted as saying by TV Station ABC23.
Jeremy needed a few stitches after Tuesday's incident but he is doing fine, Erica said.
The dog is under observation at Bakersfield Animal Care Center and will be euthanized.