Claiborne County, Tennessee Sheriff David Ray is accused of raping a 17-year-old girl while on a deer hunting trip.
The Tennessee Bureau of Investigation has also been looking into the allegations.
Ray is accused of threatening the girl with his service weapon, holding her against her will in his truck and sexually molesting her.
Ray refused to comment on the lawsuit, saying he was unaware of it.
The lawsuit was filed in US District court in Knoxville by two attorneys on behalf of the girl and her mother.
Ray, who was elected in august, is still on the job.