The boy, whose name wasn't released, was in the back yard with the dog on Sunday when it attacked.
Paramedics arrived at the home around 8 p.m. (EST) to find the boy with flesh ripped from his face and neck and wounds to his chest, arm and shoulder. He was taken to Los Angeles County-USC Medical Center in critical condition, where he died at 11:44 p.m. (EST).
An 11-year-old neighbor called for help after hearing the pit bull growling.
"I saw that (the boy) was fighting off the dog and called for his parents, but no one heard me. That's when I called my parents," Adrian Banuelos said. His father called paramedics.
The boy's mother heard the attack from the kitchen, went outside and pulled her son from the dog.
It was unclear what provoked the dog, and the family said it was the first time the animal had attacked anyone, said Officer Joe Estrada.
Animal control officers took custody of the dog.