The incident happened at about 5 p.m. Sunday at a house in Oregon City, The (Portland) Oregonian reported.
Police said the girl's mother's boyfriend was cleaning a handgun inside the house when it accidentally discharged, shooting a bullet through a wall and into the backyard where it hit the girl.
The girl was transported by helicopter to Oregon Health & Science University in Portland where she was pronounced dead.
The man who was cleaning the gun is working with police, officials said. There was no information on whether charges would be filed.
Names of those involved in the incident were not released.