The 45-year-old man was arrested yesterday (Friday) and interviewed by detectives before being released under investigation while officers continue with enquiries.
The arrest comes after an incident in East Hunsbury at about 5pm on Tuesday, November 26.
Northamptonshire Police said the girl was subjected to a serious sexual assault in a wooded area close to the junction with Towcester Road.
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Anyone with any information relating to the incident is asked to call Northamptonshire Police on 101, or Crimestoppers anonymously on 0800 555111.