The girl was walking along Whiteford Drive, Kettering at around 3.30pm on Tuesday, December 1, when the dog approached her from behind.
The dog appeared to be wondering around without an owner and did not appear to have a collar.
The dog tried to get the girls attention who continued walking without engaging with the dog.
Shortly after, the dog bit the girl causing a puncture wound to one of her fingers
The dog is described as tall and muscular and brown in colour and bigger.
The owner of the dog or anyone with any information relating to the incident can contact Northamptonshire Police on 101. Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.