Police were called to Hanemill Court in Bellinge, Northampton, shortly after 7pm yesterday.
At the scene officers discovered both the girl and her step-father had been attacked by the pet.
Duty Inspector Karl Sturman said: "At 7.13pm on Sunday, May 30 police were called to a report of a dog attack at an address in Hanemill Court.
"The family pet, a large American bull dog, had attacked a five-year-old girl. She suffered serious facial injuries.
"The injured girl's step-father also suffered injuries to his arm where he had grappled with the dog.
"Both were conveyed to hospital by ambulance for treatment."
He added the dog had been contained by police at the scene and would be put down today.
The girl is not thought to have suffered life-threatening injuries.
Northampton Chronicle & Echo
ThaIndian.com has identified the girl as Tanisha Hall.
Tanisha is described by neighbors as a "bouncy and happy five year old".
There is currently no update on Tanisha's condition.
The dog has been euthanized.
It is unknown what, if anything, triggered the attack.