Officers were called to reports of the dog attack on Westfield Drive in Preston on Sunday, July 15 at around 4.15pm.
Police say the four-year-old was reportedly playing in the garden of a house, not believed to be hers, when the dog struck, leaving her with serious injuries to her head and face.
A police spokesman said: "We were called on Sunday to reports a child had been injured by a dog.
"The four-year-old girl was taken to Royal Preston Hospital with serious injuries.
"It is thought the dog is a cross-breed and it has been seized by police."
Police say they are not currently able to confirm what type of "mixed breed" dog was involved in the incident.
A 40-year-old woman from Preston has been arrested on suspicion of having a dog dangerously out of control and remains in police custody.