Davies has signed a contract until the end of the 2011/12 season and he's taken David Kelly with him as his assistant, the same role he filled at his two previous clubs.
He doesn't start at Forest until next Monday but will watch the FA Cup tie against Manchester City this weekend with John Pemberton in charge of the team.
Chief Executive Mark Arthur said: ""Billy's record in the Championship is superb and we're hoping he can build on his already impressive achievements with us."
With Davies appointed that just leaves Blackpool and Derby in the Championship looking for new managers.